Provided by: perl-doc_5.26.2-7_all bug

NAME

       perlfunc - Perl builtin functions

DESCRIPTION

       The functions in this section can serve as terms in an expression.  They fall into two
       major categories: list operators and named unary operators.  These differ in their
       precedence relationship with a following comma.  (See the precedence table in perlop.)
       List operators take more than one argument, while unary operators can never take more than
       one argument.  Thus, a comma terminates the argument of a unary operator, but merely
       separates the arguments of a list operator.  A unary operator generally provides scalar
       context to its argument, while a list operator may provide either scalar or list contexts
       for its arguments.  If it does both, scalar arguments come first and list argument follow,
       and there can only ever be one such list argument.  For instance, "splice" has three
       scalar arguments followed by a list, whereas "gethostbyname" has four scalar arguments.

       In the syntax descriptions that follow, list operators that expect a list (and provide
       list context for elements of the list) are shown with LIST as an argument.  Such a list
       may consist of any combination of scalar arguments or list values; the list values will be
       included in the list as if each individual element were interpolated at that point in the
       list, forming a longer single-dimensional list value.  Commas should separate literal
       elements of the LIST.

       Any function in the list below may be used either with or without parentheses around its
       arguments.  (The syntax descriptions omit the parentheses.)  If you use parentheses, the
       simple but occasionally surprising rule is this: It looks like a function, therefore it is
       a function, and precedence doesn't matter.  Otherwise it's a list operator or unary
       operator, and precedence does matter.  Whitespace between the function and left
       parenthesis doesn't count, so sometimes you need to be careful:

           print 1+2+4;      # Prints 7.
           print(1+2) + 4;   # Prints 3.
           print (1+2)+4;    # Also prints 3!
           print +(1+2)+4;   # Prints 7.
           print ((1+2)+4);  # Prints 7.

       If you run Perl with the "use warnings" pragma, it can warn you about this.  For example,
       the third line above produces:

           print (...) interpreted as function at - line 1.
           Useless use of integer addition in void context at - line 1.

       A few functions take no arguments at all, and therefore work as neither unary nor list
       operators.  These include such functions as "time" and "endpwent".  For example,
       "time+86_400" always means "time() + 86_400".

       For functions that can be used in either a scalar or list context, nonabortive failure is
       generally indicated in scalar context by returning the undefined value, and in list
       context by returning the empty list.

       Remember the following important rule: There is no rule that relates the behavior of an
       expression in list context to its behavior in scalar context, or vice versa.  It might do
       two totally different things.  Each operator and function decides which sort of value
       would be most appropriate to return in scalar context.  Some operators return the length
       of the list that would have been returned in list context.  Some operators return the
       first value in the list.  Some operators return the last value in the list.  Some
       operators return a count of successful operations.  In general, they do what you want,
       unless you want consistency.

       A named array in scalar context is quite different from what would at first glance appear
       to be a list in scalar context.  You can't get a list like "(1,2,3)" into being in scalar
       context, because the compiler knows the context at compile time.  It would generate the
       scalar comma operator there, not the list concatenation version of the comma.  That means
       it was never a list to start with.

       In general, functions in Perl that serve as wrappers for system calls ("syscalls") of the
       same name (like chown(2), fork(2), closedir(2), etc.) return true when they succeed and
       "undef" otherwise, as is usually mentioned in the descriptions below.  This is different
       from the C interfaces, which return "-1" on failure.  Exceptions to this rule include
       "wait", "waitpid", and "syscall".  System calls also set the special $! variable on
       failure.  Other functions do not, except accidentally.

       Extension modules can also hook into the Perl parser to define new kinds of keyword-headed
       expression.  These may look like functions, but may also look completely different.  The
       syntax following the keyword is defined entirely by the extension.  If you are an
       implementor, see "PL_keyword_plugin" in perlapi for the mechanism.  If you are using such
       a module, see the module's documentation for details of the syntax that it defines.

   Perl Functions by Category
       Here are Perl's functions (including things that look like functions, like some keywords
       and named operators) arranged by category.  Some functions appear in more than one place.

       Functions for SCALARs or strings
           "chomp", "chop", "chr", "crypt", "fc", "hex", "index", "lc", "lcfirst", "length",
           "oct", "ord", "pack", "q//", "qq//", "reverse", "rindex", "sprintf", "substr",
           "tr///", "uc", "ucfirst", "y///"

           "fc" is available only if the "fc" feature is enabled or if it is prefixed with
           "CORE::".  The "fc" feature is enabled automatically with a "use v5.16" (or higher)
           declaration in the current scope.

       Regular expressions and pattern matching
           "m//", "pos", "qr//", "quotemeta", "s///", "split", "study"

       Numeric functions
           "abs", "atan2", "cos", "exp", "hex", "int", "log", "oct", "rand", "sin", "sqrt",
           "srand"

       Functions for real @ARRAYs
           "each", "keys", "pop", "push", "shift", "splice", "unshift", "values"

       Functions for list data
           "grep", "join", "map", "qw//", "reverse", "sort", "unpack"

       Functions for real %HASHes
           "delete", "each", "exists", "keys", "values"

       Input and output functions
           "binmode", "close", "closedir", "dbmclose", "dbmopen", "die", "eof", "fileno",
           "flock", "format", "getc", "print", "printf", "read", "readdir", "readline",
           "rewinddir", "say", "seek", "seekdir", "select", "syscall", "sysread", "sysseek",
           "syswrite", "tell", "telldir", "truncate", "warn", "write"

           "say" is available only if the "say" feature is enabled or if it is prefixed with
           "CORE::".  The "say" feature is enabled automatically with a "use v5.10" (or higher)
           declaration in the current scope.

       Functions for fixed-length data or records
           "pack", "read", "syscall", "sysread", "sysseek", "syswrite", "unpack", "vec"

       Functions for filehandles, files, or directories
           "-X", "chdir", "chmod", "chown", "chroot", "fcntl", "glob", "ioctl", "link", "lstat",
           "mkdir", "open", "opendir", "readlink", "rename", "rmdir", "select", "stat",
           "symlink", "sysopen", "umask", "unlink", "utime"

       Keywords related to the control flow of your Perl program
           "break", "caller", "continue", "die", "do", "dump", "eval", "evalbytes", "exit",
           "__FILE__", "goto", "last", "__LINE__", "next", "__PACKAGE__", "redo", "return",
           "sub", "__SUB__", "wantarray"

           "break" is available only if you enable the experimental "switch" feature or use the
           "CORE::" prefix.  The "switch" feature also enables the "default", "given" and "when"
           statements, which are documented in "Switch Statements" in perlsyn.  The "switch"
           feature is enabled automatically with a "use v5.10" (or higher) declaration in the
           current scope.  In Perl v5.14 and earlier, "continue" required the "switch" feature,
           like the other keywords.

           "evalbytes" is only available with the "evalbytes" feature (see feature) or if
           prefixed with "CORE::".  "__SUB__" is only available with the "current_sub" feature or
           if prefixed with "CORE::".  Both the "evalbytes" and "current_sub" features are
           enabled automatically with a "use v5.16" (or higher) declaration in the current scope.

       Keywords related to scoping
           "caller", "import", "local", "my", "our", "package", "state", "use"

           "state" is available only if the "state" feature is enabled or if it is prefixed with
           "CORE::".  The "state" feature is enabled automatically with a "use v5.10" (or higher)
           declaration in the current scope.

       Miscellaneous functions
           "defined", "formline", "lock", "prototype", "reset", "scalar", "undef"

       Functions for processes and process groups
           "alarm", "exec", "fork", "getpgrp", "getppid", "getpriority", "kill", "pipe", "qx//",
           "readpipe", "setpgrp", "setpriority", "sleep", "system", "times", "wait", "waitpid"

       Keywords related to Perl modules
           "do", "import", "no", "package", "require", "use"

       Keywords related to classes and object-orientation
           "bless", "dbmclose", "dbmopen", "package", "ref", "tie", "tied", "untie", "use"

       Low-level socket functions
           "accept", "bind", "connect", "getpeername", "getsockname", "getsockopt", "listen",
           "recv", "send", "setsockopt", "shutdown", "socket", "socketpair"

       System V interprocess communication functions
           "msgctl", "msgget", "msgrcv", "msgsnd", "semctl", "semget", "semop", "shmctl",
           "shmget", "shmread", "shmwrite"

       Fetching user and group info
           "endgrent", "endhostent", "endnetent", "endpwent", "getgrent", "getgrgid", "getgrnam",
           "getlogin", "getpwent", "getpwnam", "getpwuid", "setgrent", "setpwent"

       Fetching network info
           "endprotoent", "endservent", "gethostbyaddr", "gethostbyname", "gethostent",
           "getnetbyaddr", "getnetbyname", "getnetent", "getprotobyname", "getprotobynumber",
           "getprotoent", "getservbyname", "getservbyport", "getservent", "sethostent",
           "setnetent", "setprotoent", "setservent"

       Time-related functions
           "gmtime", "localtime", "time", "times"

       Non-function keywords
           "and", "AUTOLOAD", "BEGIN", "CHECK", "cmp", "CORE", "__DATA__", "default", "DESTROY",
           "else", "elseif", "elsif", "END", "__END__", "eq", "for", "foreach", "ge", "given",
           "gt", "if", "INIT", "le", "lt", "ne", "not", "or", "UNITCHECK", "unless", "until",
           "when", "while", "x", "xor"

   Portability
       Perl was born in Unix and can therefore access all common Unix system calls.  In non-Unix
       environments, the functionality of some Unix system calls may not be available or details
       of the available functionality may differ slightly.  The Perl functions affected by this
       are:

       "-X", "binmode", "chmod", "chown", "chroot", "crypt", "dbmclose", "dbmopen", "dump",
       "endgrent", "endhostent", "endnetent", "endprotoent", "endpwent", "endservent", "exec",
       "fcntl", "flock", "fork", "getgrent", "getgrgid", "gethostbyname", "gethostent",
       "getlogin", "getnetbyaddr", "getnetbyname", "getnetent", "getppid", "getpgrp",
       "getpriority", "getprotobynumber", "getprotoent", "getpwent", "getpwnam", "getpwuid",
       "getservbyport", "getservent", "getsockopt", "glob", "ioctl", "kill", "link", "lstat",
       "msgctl", "msgget", "msgrcv", "msgsnd", "open", "pipe", "readlink", "rename", "select",
       "semctl", "semget", "semop", "setgrent", "sethostent", "setnetent", "setpgrp",
       "setpriority", "setprotoent", "setpwent", "setservent", "setsockopt", "shmctl", "shmget",
       "shmread", "shmwrite", "socket", "socketpair", "stat", "symlink", "syscall", "sysopen",
       "system", "times", "truncate", "umask", "unlink", "utime", "wait", "waitpid"

       For more information about the portability of these functions, see perlport and other
       available platform-specific documentation.

   Alphabetical Listing of Perl Functions
       -X FILEHANDLE
       -X EXPR
       -X DIRHANDLE
       -X  A file test, where X is one of the letters listed below.  This unary operator takes
           one argument, either a filename, a filehandle, or a dirhandle, and tests the
           associated file to see if something is true about it.  If the argument is omitted,
           tests $_, except for "-t", which tests STDIN.  Unless otherwise documented, it returns
           1 for true and '' for false.  If the file doesn't exist or can't be examined, it
           returns "undef" and sets $! (errno).  Despite the funny names, precedence is the same
           as any other named unary operator.  The operator may be any of:

               -r  File is readable by effective uid/gid.
               -w  File is writable by effective uid/gid.
               -x  File is executable by effective uid/gid.
               -o  File is owned by effective uid.

               -R  File is readable by real uid/gid.
               -W  File is writable by real uid/gid.
               -X  File is executable by real uid/gid.
               -O  File is owned by real uid.

               -e  File exists.
               -z  File has zero size (is empty).
               -s  File has nonzero size (returns size in bytes).

               -f  File is a plain file.
               -d  File is a directory.
               -l  File is a symbolic link (false if symlinks aren't
                   supported by the file system).
               -p  File is a named pipe (FIFO), or Filehandle is a pipe.
               -S  File is a socket.
               -b  File is a block special file.
               -c  File is a character special file.
               -t  Filehandle is opened to a tty.

               -u  File has setuid bit set.
               -g  File has setgid bit set.
               -k  File has sticky bit set.

               -T  File is an ASCII or UTF-8 text file (heuristic guess).
               -B  File is a "binary" file (opposite of -T).

               -M  Script start time minus file modification time, in days.
               -A  Same for access time.
               -C  Same for inode change time (Unix, may differ for other
                   platforms)

           Example:

               while (<>) {
                   chomp;
                   next unless -f $_;  # ignore specials
                   #...
               }

           Note that "-s/a/b/" does not do a negated substitution.  Saying "-exp($foo)" still
           works as expected, however: only single letters following a minus are interpreted as
           file tests.

           These operators are exempt from the "looks like a function rule" described above.
           That is, an opening parenthesis after the operator does not affect how much of the
           following code constitutes the argument.  Put the opening parentheses before the
           operator to separate it from code that follows (this applies only to operators with
           higher precedence than unary operators, of course):

               -s($file) + 1024   # probably wrong; same as -s($file + 1024)
               (-s $file) + 1024  # correct

           The interpretation of the file permission operators "-r", "-R", "-w", "-W", "-x", and
           "-X" is by default based solely on the mode of the file and the uids and gids of the
           user.  There may be other reasons you can't actually read, write, or execute the file:
           for example network filesystem access controls, ACLs (access control lists), read-only
           filesystems, and unrecognized executable formats.  Note that the use of these six
           specific operators to verify if some operation is possible is usually a mistake,
           because it may be open to race conditions.

           Also note that, for the superuser on the local filesystems, the "-r", "-R", "-w", and
           "-W" tests always return 1, and "-x" and "-X" return 1 if any execute bit is set in
           the mode.  Scripts run by the superuser may thus need to do a "stat" to determine the
           actual mode of the file, or temporarily set their effective uid to something else.

           If you are using ACLs, there is a pragma called "filetest" that may produce more
           accurate results than the bare "stat" mode bits.  When under "use filetest 'access'",
           the above-mentioned filetests test whether the permission can(not) be granted using
           the access(2) family of system calls.  Also note that the "-x" and "-X" tests may
           under this pragma return true even if there are no execute permission bits set (nor
           any extra execute permission ACLs).  This strangeness is due to the underlying system
           calls' definitions.  Note also that, due to the implementation of "use filetest
           'access'", the "_" special filehandle won't cache the results of the file tests when
           this pragma is in effect.  Read the documentation for the "filetest" pragma for more
           information.

           The "-T" and "-B" tests work as follows.  The first block or so of the file is
           examined to see if it is valid UTF-8 that includes non-ASCII characters.  If so, it's
           a "-T" file.  Otherwise, that same portion of the file is examined for odd characters
           such as strange control codes or characters with the high bit set.  If more than a
           third of the characters are strange, it's a "-B" file; otherwise it's a "-T" file.
           Also, any file containing a zero byte in the examined portion is considered a binary
           file.  (If executed within the scope of a use locale which includes "LC_CTYPE", odd
           characters are anything that isn't a printable nor space in the current locale.)  If
           "-T" or "-B" is used on a filehandle, the current IO buffer is examined rather than
           the first block.  Both "-T" and "-B" return true on an empty file, or a file at EOF
           when testing a filehandle.  Because you have to read a file to do the "-T" test, on
           most occasions you want to use a "-f" against the file first, as in "next unless -f
           $file && -T $file".

           If any of the file tests (or either the "stat" or "lstat" operator) is given the
           special filehandle consisting of a solitary underline, then the stat structure of the
           previous file test (or "stat" operator) is used, saving a system call.  (This doesn't
           work with "-t", and you need to remember that "lstat" and "-l" leave values in the
           stat structure for the symbolic link, not the real file.)  (Also, if the stat buffer
           was filled by an "lstat" call, "-T" and "-B" will reset it with the results of "stat
           _").  Example:

               print "Can do.\n" if -r $a || -w _ || -x _;

               stat($filename);
               print "Readable\n" if -r _;
               print "Writable\n" if -w _;
               print "Executable\n" if -x _;
               print "Setuid\n" if -u _;
               print "Setgid\n" if -g _;
               print "Sticky\n" if -k _;
               print "Text\n" if -T _;
               print "Binary\n" if -B _;

           As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file test
           operators, in a way that "-f -w -x $file" is equivalent to "-x $file && -w _ && -f _".
           (This is only fancy syntax: if you use the return value of "-f $file" as an argument
           to another filetest operator, no special magic will happen.)

           Portability issues: "-X" in perlport.

           To avoid confusing would-be users of your code with mysterious syntax errors, put
           something like this at the top of your script:

               use 5.010;  # so filetest ops can stack

       abs VALUE
       abs Returns the absolute value of its argument.  If VALUE is omitted, uses $_.

       accept NEWSOCKET,GENERICSOCKET
           Accepts an incoming socket connect, just as accept(2) does.  Returns the packed
           address if it succeeded, false otherwise.  See the example in "Sockets: Client/Server
           Communication" in perlipc.

           On systems that support a close-on-exec flag on files, the flag will be set for the
           newly opened file descriptor, as determined by the value of $^F.  See "$^F" in
           perlvar.

       alarm SECONDS
       alarm
           Arranges to have a SIGALRM delivered to this process after the specified number of
           wallclock seconds has elapsed.  If SECONDS is not specified, the value stored in $_ is
           used.  (On some machines, unfortunately, the elapsed time may be up to one second less
           or more than you specified because of how seconds are counted, and process scheduling
           may delay the delivery of the signal even further.)

           Only one timer may be counting at once.  Each call disables the previous timer, and an
           argument of 0 may be supplied to cancel the previous timer without starting a new one.
           The returned value is the amount of time remaining on the previous timer.

           For delays of finer granularity than one second, the Time::HiRes module (from CPAN,
           and starting from Perl 5.8 part of the standard distribution) provides "ualarm".  You
           may also use Perl's four-argument version of "select" leaving the first three
           arguments undefined, or you might be able to use the "syscall" interface to access
           setitimer(2) if your system supports it.  See perlfaq8 for details.

           It is usually a mistake to intermix "alarm" and "sleep" calls, because "sleep" may be
           internally implemented on your system with "alarm".

           If you want to use "alarm" to time out a system call you need to use an "eval"/"die"
           pair.  You can't rely on the alarm causing the system call to fail with $! set to
           "EINTR" because Perl sets up signal handlers to restart system calls on some systems.
           Using "eval"/"die" always works, modulo the caveats given in "Signals" in perlipc.

               eval {
                   local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
                   alarm $timeout;
                   my $nread = sysread $socket, $buffer, $size;
                   alarm 0;
               };
               if ($@) {
                   die unless $@ eq "alarm\n";   # propagate unexpected errors
                   # timed out
               }
               else {
                   # didn't
               }

           For more information see perlipc.

           Portability issues: "alarm" in perlport.

       atan2 Y,X
           Returns the arctangent of Y/X in the range -PI to PI.

           For the tangent operation, you may use the "Math::Trig::tan" function, or use the
           familiar relation:

               sub tan { sin($_[0]) / cos($_[0])  }

           The return value for "atan2(0,0)" is implementation-defined; consult your atan2(3)
           manpage for more information.

           Portability issues: "atan2" in perlport.

       bind SOCKET,NAME
           Binds a network address to a socket, just as bind(2) does.  Returns true if it
           succeeded, false otherwise.  NAME should be a packed address of the appropriate type
           for the socket.  See the examples in "Sockets: Client/Server Communication" in
           perlipc.

       binmode FILEHANDLE, LAYER
       binmode FILEHANDLE
           Arranges for FILEHANDLE to be read or written in "binary" or "text" mode on systems
           where the run-time libraries distinguish between binary and text files.  If FILEHANDLE
           is an expression, the value is taken as the name of the filehandle.  Returns true on
           success, otherwise it returns "undef" and sets $! (errno).

           On some systems (in general, DOS- and Windows-based systems) "binmode" is necessary
           when you're not working with a text file.  For the sake of portability it is a good
           idea always to use it when appropriate, and never to use it when it isn't appropriate.
           Also, people can set their I/O to be by default UTF8-encoded Unicode, not bytes.

           In other words: regardless of platform, use "binmode" on binary data, like images, for
           example.

           If LAYER is present it is a single string, but may contain multiple directives.  The
           directives alter the behaviour of the filehandle.  When LAYER is present, using
           binmode on a text file makes sense.

           If LAYER is omitted or specified as ":raw" the filehandle is made suitable for passing
           binary data.  This includes turning off possible CRLF translation and marking it as
           bytes (as opposed to Unicode characters).  Note that, despite what may be implied in
           "Programming Perl" (the Camel, 3rd edition) or elsewhere, ":raw" is not simply the
           inverse of ":crlf".  Other layers that would affect the binary nature of the stream
           are also disabled.  See PerlIO, perlrun, and the discussion about the PERLIO
           environment variable.

           The ":bytes", ":crlf", ":utf8", and any other directives of the form ":...", are
           called I/O layers.  The open pragma can be used to establish default I/O layers.

           The LAYER parameter of the "binmode" function is described as "DISCIPLINE" in
           "Programming Perl, 3rd Edition".  However, since the publishing of this book, by many
           known as "Camel III", the consensus of the naming of this functionality has moved from
           "discipline" to "layer".  All documentation of this version of Perl therefore refers
           to "layers" rather than to "disciplines".  Now back to the regularly scheduled
           documentation...

           To mark FILEHANDLE as UTF-8, use ":utf8" or ":encoding(UTF-8)".  ":utf8" just marks
           the data as UTF-8 without further checking, while ":encoding(UTF-8)" checks the data
           for actually being valid UTF-8.  More details can be found in PerlIO::encoding.

           In general, "binmode" should be called after "open" but before any I/O is done on the
           filehandle.  Calling "binmode" normally flushes any pending buffered output data (and
           perhaps pending input data) on the handle.  An exception to this is the ":encoding"
           layer that changes the default character encoding of the handle.  The ":encoding"
           layer sometimes needs to be called in mid-stream, and it doesn't flush the stream.
           ":encoding" also implicitly pushes on top of itself the ":utf8" layer because
           internally Perl operates on UTF8-encoded Unicode characters.

           The operating system, device drivers, C libraries, and Perl run-time system all
           conspire to let the programmer treat a single character ("\n") as the line terminator,
           irrespective of external representation.  On many operating systems, the native text
           file representation matches the internal representation, but on some platforms the
           external representation of "\n" is made up of more than one character.

           All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use a single
           character to end each line in the external representation of text (even though that
           single character is CARRIAGE RETURN on old, pre-Darwin flavors of Mac OS, and is LINE
           FEED on Unix and most VMS files).  In other systems like OS/2, DOS, and the various
           flavors of MS-Windows, your program sees a "\n" as a simple "\cJ", but what's stored
           in text files are the two characters "\cM\cJ".  That means that if you don't use
           "binmode" on these systems, "\cM\cJ" sequences on disk will be converted to "\n" on
           input, and any "\n" in your program will be converted back to "\cM\cJ" on output.
           This is what you want for text files, but it can be disastrous for binary files.

           Another consequence of using "binmode" (on some systems) is that special end-of-file
           markers will be seen as part of the data stream.  For systems from the Microsoft
           family this means that, if your binary data contain "\cZ", the I/O subsystem will
           regard it as the end of the file, unless you use "binmode".

           "binmode" is important not only for "readline" and "print" operations, but also when
           using "read", "seek", "sysread", "syswrite" and "tell" (see perlport for more
           details).  See the $/ and "$\" variables in perlvar for how to manually set your input
           and output line-termination sequences.

           Portability issues: "binmode" in perlport.

       bless REF,CLASSNAME
       bless REF
           This function tells the thingy referenced by REF that it is now an object in the
           CLASSNAME package.  If CLASSNAME is omitted, the current package is used.  Because a
           "bless" is often the last thing in a constructor, it returns the reference for
           convenience.  Always use the two-argument version if a derived class might inherit the
           method doing the blessing.  See perlobj for more about the blessing (and blessings) of
           objects.

           Consider always blessing objects in CLASSNAMEs that are mixed case.  Namespaces with
           all lowercase names are considered reserved for Perl pragmas.  Builtin types have all
           uppercase names.  To prevent confusion, you may wish to avoid such package names as
           well.  Make sure that CLASSNAME is a true value.

           See "Perl Modules" in perlmod.

       break
           Break out of a "given" block.

           "break" is available only if the "switch" feature is enabled or if it is prefixed with
           "CORE::". The "switch" feature is enabled automatically with a "use v5.10" (or higher)
           declaration in the current scope.

       caller EXPR
       caller
           Returns the context of the current pure perl subroutine call.  In scalar context,
           returns the caller's package name if there is a caller (that is, if we're in a
           subroutine or "eval" or "require") and the undefined value otherwise.  caller never
           returns XS subs and they are skipped.  The next pure perl sub will appear instead of
           the XS sub in caller's return values.  In list context, caller returns

                  # 0         1          2
               my ($package, $filename, $line) = caller;

           With EXPR, it returns some extra information that the debugger uses to print a stack
           trace.  The value of EXPR indicates how many call frames to go back before the current
           one.

               #  0         1          2      3            4
            my ($package, $filename, $line, $subroutine, $hasargs,

               #  5          6          7            8       9         10
               $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
             = caller($i);

           Here, $subroutine is the function that the caller called (rather than the function
           containing the caller).  Note that $subroutine may be "(eval)" if the frame is not a
           subroutine call, but an "eval".  In such a case additional elements $evaltext and
           $is_require are set: $is_require is true if the frame is created by a "require" or
           "use" statement, $evaltext contains the text of the "eval EXPR" statement.  In
           particular, for an "eval BLOCK" statement, $subroutine is "(eval)", but $evaltext is
           undefined.  (Note also that each "use" statement creates a "require" frame inside an
           "eval EXPR" frame.)  $subroutine may also be "(unknown)" if this particular subroutine
           happens to have been deleted from the symbol table.  $hasargs is true if a new
           instance of @_ was set up for the frame.  $hints and $bitmask contain pragmatic hints
           that the caller was compiled with.  $hints corresponds to $^H, and $bitmask
           corresponds to "${^WARNING_BITS}".  The $hints and $bitmask values are subject to
           change between versions of Perl, and are not meant for external use.

           $hinthash is a reference to a hash containing the value of "%^H" when the caller was
           compiled, or "undef" if "%^H" was empty.  Do not modify the values of this hash, as
           they are the actual values stored in the optree.

           Furthermore, when called from within the DB package in list context, and with an
           argument, caller returns more detailed information: it sets the list variable
           @DB::args to be the arguments with which the subroutine was invoked.

           Be aware that the optimizer might have optimized call frames away before "caller" had
           a chance to get the information.  That means that caller(N) might not return
           information about the call frame you expect it to, for "N > 1".  In particular,
           @DB::args might have information from the previous time "caller" was called.

           Be aware that setting @DB::args is best effort, intended for debugging or generating
           backtraces, and should not be relied upon.  In particular, as @_ contains aliases to
           the caller's arguments, Perl does not take a copy of @_, so @DB::args will contain
           modifications the subroutine makes to @_ or its contents, not the original values at
           call time.  @DB::args, like @_, does not hold explicit references to its elements, so
           under certain cases its elements may have become freed and reallocated for other
           variables or temporary values.  Finally, a side effect of the current implementation
           is that the effects of "shift @_" can normally be undone (but not "pop @_" or other
           splicing, and not if a reference to @_ has been taken, and subject to the caveat about
           reallocated elements), so @DB::args is actually a hybrid of the current state and
           initial state of @_.  Buyer beware.

       chdir EXPR
       chdir FILEHANDLE
       chdir DIRHANDLE
       chdir
           Changes the working directory to EXPR, if possible.  If EXPR is omitted, changes to
           the directory specified by $ENV{HOME}, if set; if not, changes to the directory
           specified by $ENV{LOGDIR}.  (Under VMS, the variable $ENV{'SYS$LOGIN'} is also
           checked, and used if it is set.)  If neither is set, "chdir" does nothing and fails.
           It returns true on success, false otherwise.  See the example under "die".

           On systems that support fchdir(2), you may pass a filehandle or directory handle as
           the argument.  On systems that don't support fchdir(2), passing handles raises an
           exception.

       chmod LIST
           Changes the permissions of a list of files.  The first element of the list must be the
           numeric mode, which should probably be an octal number, and which definitely should
           not be a string of octal digits: 0644 is okay, but "0644" is not.  Returns the number
           of files successfully changed.  See also "oct" if all you have is a string.

               my $cnt = chmod 0755, "foo", "bar";
               chmod 0755, @executables;
               my $mode = "0644"; chmod $mode, "foo";      # !!! sets mode to
                                                           # --w----r-T
               my $mode = "0644"; chmod oct($mode), "foo"; # this is better
               my $mode = 0644;   chmod $mode, "foo";      # this is best

           On systems that support fchmod(2), you may pass filehandles among the files.  On
           systems that don't support fchmod(2), passing filehandles raises an exception.
           Filehandles must be passed as globs or glob references to be recognized; barewords are
           considered filenames.

               open(my $fh, "<", "foo");
               my $perm = (stat $fh)[2] & 07777;
               chmod($perm | 0600, $fh);

           You can also import the symbolic "S_I*" constants from the "Fcntl" module:

               use Fcntl qw( :mode );
               chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
               # Identical to the chmod 0755 of the example above.

           Portability issues: "chmod" in perlport.

       chomp VARIABLE
       chomp( LIST )
       chomp
           This safer version of "chop" removes any trailing string that corresponds to the
           current value of $/ (also known as $INPUT_RECORD_SEPARATOR in the "English" module).
           It returns the total number of characters removed from all its arguments.  It's often
           used to remove the newline from the end of an input record when you're worried that
           the final record may be missing its newline.  When in paragraph mode ("$/ = ''"), it
           removes all trailing newlines from the string.  When in slurp mode ("$/ = undef") or
           fixed-length record mode ($/ is a reference to an integer or the like; see perlvar),
           "chomp" won't remove anything.  If VARIABLE is omitted, it chomps $_.  Example:

               while (<>) {
                   chomp;  # avoid \n on last field
                   my @array = split(/:/);
                   # ...
               }

           If VARIABLE is a hash, it chomps the hash's values, but not its keys, resetting the
           "each" iterator in the process.

           You can actually chomp anything that's an lvalue, including an assignment:

               chomp(my $cwd = `pwd`);
               chomp(my $answer = <STDIN>);

           If you chomp a list, each element is chomped, and the total number of characters
           removed is returned.

           Note that parentheses are necessary when you're chomping anything that is not a simple
           variable.  This is because "chomp $cwd = `pwd`;" is interpreted as "(chomp $cwd) =
           `pwd`;", rather than as "chomp( $cwd = `pwd` )" which you might expect.  Similarly,
           "chomp $a, $b" is interpreted as "chomp($a), $b" rather than as "chomp($a, $b)".

       chop VARIABLE
       chop( LIST )
       chop
           Chops off the last character of a string and returns the character chopped.  It is
           much more efficient than "s/.$//s" because it neither scans nor copies the string.  If
           VARIABLE is omitted, chops $_.  If VARIABLE is a hash, it chops the hash's values, but
           not its keys, resetting the "each" iterator in the process.

           You can actually chop anything that's an lvalue, including an assignment.

           If you chop a list, each element is chopped.  Only the value of the last "chop" is
           returned.

           Note that "chop" returns the last character.  To return all but the last character,
           use "substr($string, 0, -1)".

           See also "chomp".

       chown LIST
           Changes the owner (and group) of a list of files.  The first two elements of the list
           must be the numeric uid and gid, in that order.  A value of -1 in either position is
           interpreted by most systems to leave that value unchanged.  Returns the number of
           files successfully changed.

               my $cnt = chown $uid, $gid, 'foo', 'bar';
               chown $uid, $gid, @filenames;

           On systems that support fchown(2), you may pass filehandles among the files.  On
           systems that don't support fchown(2), passing filehandles raises an exception.
           Filehandles must be passed as globs or glob references to be recognized; barewords are
           considered filenames.

           Here's an example that looks up nonnumeric uids in the passwd file:

               print "User: ";
               chomp(my $user = <STDIN>);
               print "Files: ";
               chomp(my $pattern = <STDIN>);

               my ($login,$pass,$uid,$gid) = getpwnam($user)
                   or die "$user not in passwd file";

               my @ary = glob($pattern);  # expand filenames
               chown $uid, $gid, @ary;

           On most systems, you are not allowed to change the ownership of the file unless you're
           the superuser, although you should be able to change the group to any of your
           secondary groups.  On insecure systems, these restrictions may be relaxed, but this is
           not a portable assumption.  On POSIX systems, you can detect this condition this way:

               use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
               my $can_chown_giveaway = ! sysconf(_PC_CHOWN_RESTRICTED);

           Portability issues: "chown" in perlport.

       chr NUMBER
       chr Returns the character represented by that NUMBER in the character set.  For example,
           "chr(65)" is "A" in either ASCII or Unicode, and chr(0x263a) is a Unicode smiley face.

           Negative values give the Unicode replacement character (chr(0xfffd)), except under the
           bytes pragma, where the low eight bits of the value (truncated to an integer) are
           used.

           If NUMBER is omitted, uses $_.

           For the reverse, use "ord".

           Note that characters from 128 to 255 (inclusive) are by default internally not encoded
           as UTF-8 for backward compatibility reasons.

           See perlunicode for more about Unicode.

       chroot FILENAME
       chroot
           This function works like the system call by the same name: it makes the named
           directory the new root directory for all further pathnames that begin with a "/" by
           your process and all its children.  (It doesn't change your current working directory,
           which is unaffected.)  For security reasons, this call is restricted to the superuser.
           If FILENAME is omitted, does a "chroot" to $_.

           NOTE:  It is good security practice to do "chdir("/")" ("chdir" to the root directory)
           immediately after a "chroot".

           Portability issues: "chroot" in perlport.

       close FILEHANDLE
       close
           Closes the file or pipe associated with the filehandle, flushes the IO buffers, and
           closes the system file descriptor.  Returns true if those operations succeed and if no
           error was reported by any PerlIO layer.  Closes the currently selected filehandle if
           the argument is omitted.

           You don't have to close FILEHANDLE if you are immediately going to do another "open"
           on it, because "open" closes it for you.  (See "open".) However, an explicit "close"
           on an input file resets the line counter ($.), while the implicit close done by "open"
           does not.

           If the filehandle came from a piped open, "close" returns false if one of the other
           syscalls involved fails or if its program exits with non-zero status.  If the only
           problem was that the program exited non-zero, $! will be set to 0.  Closing a pipe
           also waits for the process executing on the pipe to exit--in case you wish to look at
           the output of the pipe afterwards--and implicitly puts the exit status value of that
           command into $? and "${^CHILD_ERROR_NATIVE}".

           If there are multiple threads running, "close" on a filehandle from a piped open
           returns true without waiting for the child process to terminate, if the filehandle is
           still open in another thread.

           Closing the read end of a pipe before the process writing to it at the other end is
           done writing results in the writer receiving a SIGPIPE.  If the other end can't handle
           that, be sure to read all the data before closing the pipe.

           Example:

               open(OUTPUT, '|sort >foo')  # pipe to sort
                   or die "Can't start sort: $!";
               #...                        # print stuff to output
               close OUTPUT                # wait for sort to finish
                   or warn $! ? "Error closing sort pipe: $!"
                              : "Exit status $? from sort";
               open(INPUT, 'foo')          # get sort's results
                   or die "Can't open 'foo' for input: $!";

           FILEHANDLE may be an expression whose value can be used as an indirect filehandle,
           usually the real filehandle name or an autovivified handle.

       closedir DIRHANDLE
           Closes a directory opened by "opendir" and returns the success of that system call.

       connect SOCKET,NAME
           Attempts to connect to a remote socket, just like connect(2).  Returns true if it
           succeeded, false otherwise.  NAME should be a packed address of the appropriate type
           for the socket.  See the examples in "Sockets: Client/Server Communication" in
           perlipc.

       continue BLOCK
       continue
           When followed by a BLOCK, "continue" is actually a flow control statement rather than
           a function.  If there is a "continue" BLOCK attached to a BLOCK (typically in a
           "while" or "foreach"), it is always executed just before the conditional is about to
           be evaluated again, just like the third part of a "for" loop in C.  Thus it can be
           used to increment a loop variable, even when the loop has been continued via the
           "next" statement (which is similar to the C "continue" statement).

           "last", "next", or "redo" may appear within a "continue" block; "last" and "redo"
           behave as if they had been executed within the main block.  So will "next", but since
           it will execute a "continue" block, it may be more entertaining.

               while (EXPR) {
                   ### redo always comes here
                   do_something;
               } continue {
                   ### next always comes here
                   do_something_else;
                   # then back the top to re-check EXPR
               }
               ### last always comes here

           Omitting the "continue" section is equivalent to using an empty one, logically enough,
           so "next" goes directly back to check the condition at the top of the loop.

           When there is no BLOCK, "continue" is a function that falls through the current "when"
           or "default" block instead of iterating a dynamically enclosing "foreach" or exiting a
           lexically enclosing "given".  In Perl 5.14 and earlier, this form of "continue" was
           only available when the "switch" feature was enabled.  See feature and "Switch
           Statements" in perlsyn for more information.

       cos EXPR
       cos Returns the cosine of EXPR (expressed in radians).  If EXPR is omitted, takes the
           cosine of $_.

           For the inverse cosine operation, you may use the "Math::Trig::acos" function, or use
           this relation:

               sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }

       crypt PLAINTEXT,SALT
           Creates a digest string exactly like the crypt(3) function in the C library (assuming
           that you actually have a version there that has not been extirpated as a potential
           munition).

           "crypt" is a one-way hash function.  The PLAINTEXT and SALT are turned into a short
           string, called a digest, which is returned.  The same PLAINTEXT and SALT will always
           return the same string, but there is no (known) way to get the original PLAINTEXT from
           the hash.  Small changes in the PLAINTEXT or SALT will result in large changes in the
           digest.

           There is no decrypt function.  This function isn't all that useful for cryptography
           (for that, look for Crypt modules on your nearby CPAN mirror) and the name "crypt" is
           a bit of a misnomer.  Instead it is primarily used to check if two pieces of text are
           the same without having to transmit or store the text itself.  An example is checking
           if a correct password is given.  The digest of the password is stored, not the
           password itself.  The user types in a password that is "crypt"'d with the same salt as
           the stored digest.  If the two digests match, the password is correct.

           When verifying an existing digest string you should use the digest as the salt (like
           "crypt($plain, $digest) eq $digest").  The SALT used to create the digest is visible
           as part of the digest.  This ensures "crypt" will hash the new string with the same
           salt as the digest.  This allows your code to work with the standard "crypt" and with
           more exotic implementations.  In other words, assume nothing about the returned string
           itself nor about how many bytes of SALT may matter.

           Traditionally the result is a string of 13 bytes: two first bytes of the salt,
           followed by 11 bytes from the set "[./0-9A-Za-z]", and only the first eight bytes of
           PLAINTEXT mattered.  But alternative hashing schemes (like MD5), higher level security
           schemes (like C2), and implementations on non-Unix platforms may produce different
           strings.

           When choosing a new salt create a random two character string whose characters come
           from the set "[./0-9A-Za-z]" (like "join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand
           64, rand 64]").  This set of characters is just a recommendation; the characters
           allowed in the salt depend solely on your system's crypt library, and Perl can't
           restrict what salts "crypt" accepts.

           Here's an example that makes sure that whoever runs this program knows their password:

               my $pwd = (getpwuid($<))[1];

               system "stty -echo";
               print "Password: ";
               chomp(my $word = <STDIN>);
               print "\n";
               system "stty echo";

               if (crypt($word, $pwd) ne $pwd) {
                   die "Sorry...\n";
               } else {
                   print "ok\n";
               }

           Of course, typing in your own password to whoever asks you for it is unwise.

           The "crypt" function is unsuitable for hashing large quantities of data, not least of
           all because you can't get the information back.  Look at the Digest module for more
           robust algorithms.

           If using "crypt" on a Unicode string (which potentially has characters with codepoints
           above 255), Perl tries to make sense of the situation by trying to downgrade (a copy
           of) the string back to an eight-bit byte string before calling "crypt" (on that copy).
           If that works, good.  If not, "crypt" dies with "Wide character in crypt".

           Portability issues: "crypt" in perlport.

       dbmclose HASH
           [This function has been largely superseded by the "untie" function.]

           Breaks the binding between a DBM file and a hash.

           Portability issues: "dbmclose" in perlport.

       dbmopen HASH,DBNAME,MASK
           [This function has been largely superseded by the "tie" function.]

           This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a hash.  HASH
           is the name of the hash.  (Unlike normal "open", the first argument is not a
           filehandle, even though it looks like one).  DBNAME is the name of the database
           (without the .dir or .pag extension if any).  If the database does not exist, it is
           created with protection specified by MASK (as modified by the "umask").  To prevent
           creation of the database if it doesn't exist, you may specify a MODE of 0, and the
           function will return a false value if it can't find an existing database.  If your
           system supports only the older DBM functions, you may make only one "dbmopen" call in
           your program.  In older versions of Perl, if your system had neither DBM nor ndbm,
           calling "dbmopen" produced a fatal error; it now falls back to sdbm(3).

           If you don't have write access to the DBM file, you can only read hash variables, not
           set them.  If you want to test whether you can write, either use file tests or try
           setting a dummy hash entry inside an "eval" to trap the error.

           Note that functions such as "keys" and "values" may return huge lists when used on
           large DBM files.  You may prefer to use the "each" function to iterate over large DBM
           files.  Example:

               # print out history file offsets
               dbmopen(%HIST,'/usr/lib/news/history',0666);
               while (($key,$val) = each %HIST) {
                   print $key, ' = ', unpack('L',$val), "\n";
               }
               dbmclose(%HIST);

           See also AnyDBM_File for a more general description of the pros and cons of the
           various dbm approaches, as well as DB_File for a particularly rich implementation.

           You can control which DBM library you use by loading that library before you call
           "dbmopen":

               use DB_File;
               dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
                   or die "Can't open netscape history file: $!";

           Portability issues: "dbmopen" in perlport.

       defined EXPR
       defined
           Returns a Boolean value telling whether EXPR has a value other than the undefined
           value "undef".  If EXPR is not present, $_ is checked.

           Many operations return "undef" to indicate failure, end of file, system error,
           uninitialized variable, and other exceptional conditions.  This function allows you to
           distinguish "undef" from other values.  (A simple Boolean test will not distinguish
           among "undef", zero, the empty string, and "0", which are all equally false.)  Note
           that since "undef" is a valid scalar, its presence doesn't necessarily indicate an
           exceptional condition: "pop" returns "undef" when its argument is an empty array, or
           when the element to return happens to be "undef".

           You may also use "defined(&func)" to check whether subroutine "func" has ever been
           defined.  The return value is unaffected by any forward declarations of "func".  A
           subroutine that is not defined may still be callable: its package may have an
           "AUTOLOAD" method that makes it spring into existence the first time that it is
           called; see perlsub.

           Use of "defined" on aggregates (hashes and arrays) is no longer supported. It used to
           report whether memory for that aggregate had ever been allocated.  You should instead
           use a simple test for size:

               if (@an_array) { print "has array elements\n" }
               if (%a_hash)   { print "has hash members\n"   }

           When used on a hash element, it tells you whether the value is defined, not whether
           the key exists in the hash.  Use "exists" for the latter purpose.

           Examples:

               print if defined $switch{D};
               print "$val\n" while defined($val = pop(@ary));
               die "Can't readlink $sym: $!"
                   unless defined($value = readlink $sym);
               sub foo { defined &$bar ? $bar->(@_) : die "No bar"; }
               $debugging = 0 unless defined $debugging;

           Note:  Many folks tend to overuse "defined" and are then surprised to discover that
           the number 0 and "" (the zero-length string) are, in fact, defined values.  For
           example, if you say

               "ab" =~ /a(.*)b/;

           The pattern match succeeds and $1 is defined, although it matched "nothing".  It
           didn't really fail to match anything.  Rather, it matched something that happened to
           be zero characters long.  This is all very above-board and honest.  When a function
           returns an undefined value, it's an admission that it couldn't give you an honest
           answer.  So you should use "defined" only when questioning the integrity of what
           you're trying to do.  At other times, a simple comparison to 0 or "" is what you want.

           See also "undef", "exists", "ref".

       delete EXPR
           Given an expression that specifies an element or slice of a hash, "delete" deletes the
           specified elements from that hash so that "exists" on that element no longer returns
           true.  Setting a hash element to the undefined value does not remove its key, but
           deleting it does; see "exists".

           In list context, returns the value or values deleted, or the last such element in
           scalar context.  The return list's length always matches that of the argument list:
           deleting non-existent elements returns the undefined value in their corresponding
           positions.

           "delete" may also be used on arrays and array slices, but its behavior is less
           straightforward.  Although "exists" will return false for deleted entries, deleting
           array elements never changes indices of existing values; use "shift" or "splice" for
           that.  However, if any deleted elements fall at the end of an array, the array's size
           shrinks to the position of the highest element that still tests true for "exists", or
           to 0 if none do.  In other words, an array won't have trailing nonexistent elements
           after a delete.

           WARNING: Calling "delete" on array values is strongly discouraged.  The notion of
           deleting or checking the existence of Perl array elements is not conceptually
           coherent, and can lead to surprising behavior.

           Deleting from %ENV modifies the environment.  Deleting from a hash tied to a DBM file
           deletes the entry from the DBM file.  Deleting from a "tied" hash or array may not
           necessarily return anything; it depends on the implementation of the "tied" package's
           DELETE method, which may do whatever it pleases.

           The "delete local EXPR" construct localizes the deletion to the current block at run
           time.  Until the block exits, elements locally deleted temporarily no longer exist.
           See "Localized deletion of elements of composite types" in perlsub.

               my %hash = (foo => 11, bar => 22, baz => 33);
               my $scalar = delete $hash{foo};         # $scalar is 11
               $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
               my @array  = delete @hash{qw(foo baz)}; # @array  is (undef,33)

           The following (inefficiently) deletes all the values of %HASH and @ARRAY:

               foreach my $key (keys %HASH) {
                   delete $HASH{$key};
               }

               foreach my $index (0 .. $#ARRAY) {
                   delete $ARRAY[$index];
               }

           And so do these:

               delete @HASH{keys %HASH};

               delete @ARRAY[0 .. $#ARRAY];

           But both are slower than assigning the empty list or undefining %HASH or @ARRAY, which
           is the customary way to empty out an aggregate:

               %HASH = ();     # completely empty %HASH
               undef %HASH;    # forget %HASH ever existed

               @ARRAY = ();    # completely empty @ARRAY
               undef @ARRAY;   # forget @ARRAY ever existed

           The EXPR can be arbitrarily complicated provided its final operation is an element or
           slice of an aggregate:

               delete $ref->[$x][$y]{$key};
               delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};

               delete $ref->[$x][$y][$index];
               delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];

       die LIST
           "die" raises an exception.  Inside an "eval" the error message is stuffed into $@ and
           the "eval" is terminated with the undefined value.  If the exception is outside of all
           enclosing "eval"s, then the uncaught exception prints LIST to "STDERR" and exits with
           a non-zero value.  If you need to exit the process with a specific exit code, see
           "exit".

           Equivalent examples:

               die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
               chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"

           If the last element of LIST does not end in a newline, the current script line number
           and input line number (if any) are also printed, and a newline is supplied.  Note that
           the "input line number" (also known as "chunk") is subject to whatever notion of
           "line" happens to be currently in effect, and is also available as the special
           variable $..  See "$/" in perlvar and "$." in perlvar.

           Hint: sometimes appending ", stopped" to your message will cause it to make better
           sense when the string "at foo line 123" is appended.  Suppose you are running script
           "canasta".

               die "/etc/games is no good";
               die "/etc/games is no good, stopped";

           produce, respectively

               /etc/games is no good at canasta line 123.
               /etc/games is no good, stopped at canasta line 123.

           If the output is empty and $@ already contains a value (typically from a previous
           "eval") that value is reused after appending "\t...propagated".  This is useful for
           propagating exceptions:

               eval { ... };
               die unless $@ =~ /Expected exception/;

           If the output is empty and $@ contains an object reference that has a "PROPAGATE"
           method, that method will be called with additional file and line number parameters.
           The return value replaces the value in $@;  i.e., as if "$@ = eval {
           $@->PROPAGATE(__FILE__, __LINE__) };" were called.

           If $@ is empty, then the string "Died" is used.

           If an uncaught exception results in interpreter exit, the exit code is determined from
           the values of $! and $? with this pseudocode:

               exit $! if $!;              # errno
               exit $? >> 8 if $? >> 8;    # child exit status
               exit 255;                   # last resort

           As with "exit", $? is set prior to unwinding the call stack; any "DESTROY" or "END"
           handlers can then alter this value, and thus Perl's exit code.

           The intent is to squeeze as much possible information about the likely cause into the
           limited space of the system exit code.  However, as $! is the value of C's "errno",
           which can be set by any system call, this means that the value of the exit code used
           by "die" can be non-predictable, so should not be relied upon, other than to be non-
           zero.

           You can also call "die" with a reference argument, and if this is trapped within an
           "eval", $@ contains that reference.  This permits more elaborate exception handling
           using objects that maintain arbitrary state about the exception.  Such a scheme is
           sometimes preferable to matching particular string values of $@ with regular
           expressions.  Because $@ is a global variable and "eval" may be used within object
           implementations, be careful that analyzing the error object doesn't replace the
           reference in the global variable.  It's easiest to make a local copy of the reference
           before any manipulations.  Here's an example:

               use Scalar::Util "blessed";

               eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
               if (my $ev_err = $@) {
                   if (blessed($ev_err)
                       && $ev_err->isa("Some::Module::Exception")) {
                       # handle Some::Module::Exception
                   }
                   else {
                       # handle all other possible exceptions
                   }
               }

           Because Perl stringifies uncaught exception messages before display, you'll probably
           want to overload stringification operations on exception objects.  See overload for
           details about that.

           You can arrange for a callback to be run just before the "die" does its deed, by
           setting the $SIG{__DIE__} hook.  The associated handler is called with the error text
           and can change the error message, if it sees fit, by calling "die" again.  See "%SIG"
           in perlvar for details on setting %SIG entries, and "eval" for some examples.
           Although this feature was to be run only right before your program was to exit, this
           is not currently so: the $SIG{__DIE__} hook is currently called even inside "eval"ed
           blocks/strings!  If one wants the hook to do nothing in such situations, put

               die @_ if $^S;

           as the first line of the handler (see "$^S" in perlvar).  Because this promotes
           strange action at a distance, this counterintuitive behavior may be fixed in a future
           release.

           See also "exit", "warn", and the Carp module.

       do BLOCK
           Not really a function.  Returns the value of the last command in the sequence of
           commands indicated by BLOCK.  When modified by the "while" or "until" loop modifier,
           executes the BLOCK once before testing the loop condition.  (On other statements the
           loop modifiers test the conditional first.)

           "do BLOCK" does not count as a loop, so the loop control statements "next", "last", or
           "redo" cannot be used to leave or restart the block.  See perlsyn for alternative
           strategies.

       do EXPR
           Uses the value of EXPR as a filename and executes the contents of the file as a Perl
           script:

               # load the exact specified file (./ and ../ special-cased)
               do '/foo/stat.pl';
               do './stat.pl';
               do '../foo/stat.pl';

               # search for the named file within @INC
               do 'stat.pl';
               do 'foo/stat.pl';

           "do './stat.pl'" is largely like

               eval `cat stat.pl`;

           except that it's more concise, runs no external processes, and keeps track of the
           current filename for error messages. It also differs in that code evaluated with "do
           FILE" cannot see lexicals in the enclosing scope; "eval STRING" does.  It's the same,
           however, in that it does reparse the file every time you call it, so you probably
           don't want to do this inside a loop.

           Using "do" with a relative path (except for ./ and ../), like

               do 'foo/stat.pl';

           will search the @INC directories, and update %INC if the file is found.  See "@INC" in
           perlvar and "%INC" in perlvar for these variables. In particular, note that whilst
           historically @INC contained '.' (the current directory) making these two cases
           equivalent, that is no longer necessarily the case, as '.' is not included in @INC by
           default in perl versions 5.26.0 onwards. Instead, perl will now warn:

               do "stat.pl" failed, '.' is no longer in @INC;
               did you mean do "./stat.pl"?

           If "do" can read the file but cannot compile it, it returns "undef" and sets an error
           message in $@.  If "do" cannot read the file, it returns undef and sets $! to the
           error.  Always check $@ first, as compilation could fail in a way that also sets $!.
           If the file is successfully compiled, "do" returns the value of the last expression
           evaluated.

           Inclusion of library modules is better done with the "use" and "require" operators,
           which also do automatic error checking and raise an exception if there's a problem.

           You might like to use "do" to read in a program configuration file.  Manual error
           checking can be done this way:

               # Read in config files: system first, then user.
               # Beware of using relative pathnames here.
               for $file ("/share/prog/defaults.rc",
                          "$ENV{HOME}/.someprogrc")
               {
                   unless ($return = do $file) {
                       warn "couldn't parse $file: $@" if $@;
                       warn "couldn't do $file: $!"    unless defined $return;
                       warn "couldn't run $file"       unless $return;
                   }
               }

       dump LABEL
       dump EXPR
       dump
           This function causes an immediate core dump.  See also the -u command-line switch in
           perlrun, which does the same thing.  Primarily this is so that you can use the undump
           program (not supplied) to turn your core dump into an executable binary after having
           initialized all your variables at the beginning of the program.  When the new binary
           is executed it will begin by executing a "goto LABEL" (with all the restrictions that
           "goto" suffers).  Think of it as a goto with an intervening core dump and
           reincarnation.  If "LABEL" is omitted, restarts the program from the top.  The "dump
           EXPR" form, available starting in Perl 5.18.0, allows a name to be computed at run
           time, being otherwise identical to "dump LABEL".

           WARNING: Any files opened at the time of the dump will not be open any more when the
           program is reincarnated, with possible resulting confusion by Perl.

           This function is now largely obsolete, mostly because it's very hard to convert a core
           file into an executable.  That's why you should now invoke it as "CORE::dump()" if you
           don't want to be warned against a possible typo.

           Unlike most named operators, this has the same precedence as assignment.  It is also
           exempt from the looks-like-a-function rule, so "dump ("foo")."bar"" will cause "bar"
           to be part of the argument to "dump".

           Portability issues: "dump" in perlport.

       each HASH
       each ARRAY
           When called on a hash in list context, returns a 2-element list consisting of the key
           and value for the next element of a hash.  In Perl 5.12 and later only, it will also
           return the index and value for the next element of an array so that you can iterate
           over it; older Perls consider this a syntax error.  When called in scalar context,
           returns only the key (not the value) in a hash, or the index in an array.

           Hash entries are returned in an apparently random order.  The actual random order is
           specific to a given hash; the exact same series of operations on two hashes may result
           in a different order for each hash.  Any insertion into the hash may change the order,
           as will any deletion, with the exception that the most recent key returned by "each"
           or "keys" may be deleted without changing the order.  So long as a given hash is
           unmodified you may rely on "keys", "values" and "each" to repeatedly return the same
           order as each other.  See "Algorithmic Complexity Attacks" in perlsec for details on
           why hash order is randomized.  Aside from the guarantees provided here the exact
           details of Perl's hash algorithm and the hash traversal order are subject to change in
           any release of Perl.

           After "each" has returned all entries from the hash or array, the next call to "each"
           returns the empty list in list context and "undef" in scalar context; the next call
           following that one restarts iteration.  Each hash or array has its own internal
           iterator, accessed by "each", "keys", and "values".  The iterator is implicitly reset
           when "each" has reached the end as just described; it can be explicitly reset by
           calling "keys" or "values" on the hash or array.  If you add or delete a hash's
           elements while iterating over it, the effect on the iterator is unspecified; for
           example, entries may be skipped or duplicated--so don't do that.  Exception: It is
           always safe to delete the item most recently returned by "each", so the following code
           works properly:

               while (my ($key, $value) = each %hash) {
                   print $key, "\n";
                   delete $hash{$key};   # This is safe
               }

           Tied hashes may have a different ordering behaviour to perl's hash implementation.

           This prints out your environment like the printenv(1) program, but in a different
           order:

               while (my ($key,$value) = each %ENV) {
                   print "$key=$value\n";
               }

           Starting with Perl 5.14, an experimental feature allowed "each" to take a scalar
           expression. This experiment has been deemed unsuccessful, and was removed as of Perl
           5.24.

           As of Perl 5.18 you can use a bare "each" in a "while" loop, which will set $_ on
           every iteration.

               while (each %ENV) {
                   print "$_=$ENV{$_}\n";
               }

           To avoid confusing would-be users of your code who are running earlier versions of
           Perl with mysterious syntax errors, put this sort of thing at the top of your file to
           signal that your code will work only on Perls of a recent vintage:

               use 5.012;  # so keys/values/each work on arrays
               use 5.018;  # so each assigns to $_ in a lone while test

           See also "keys", "values", and "sort".

       eof FILEHANDLE
       eof ()
       eof Returns 1 if the next read on FILEHANDLE will return end of file or if FILEHANDLE is
           not open.  FILEHANDLE may be an expression whose value gives the real filehandle.
           (Note that this function actually reads a character and then "ungetc"s it, so isn't
           useful in an interactive context.)  Do not read from a terminal file (or call
           "eof(FILEHANDLE)" on it) after end-of-file is reached.  File types such as terminals
           may lose the end-of-file condition if you do.

           An "eof" without an argument uses the last file read.  Using "eof()" with empty
           parentheses is different.  It refers to the pseudo file formed from the files listed
           on the command line and accessed via the "<>" operator.  Since "<>" isn't explicitly
           opened, as a normal filehandle is, an "eof()" before "<>" has been used will cause
           @ARGV to be examined to determine if input is available.   Similarly, an "eof()" after
           "<>" has returned end-of-file will assume you are processing another @ARGV list, and
           if you haven't set @ARGV, will read input from "STDIN"; see "I/O Operators" in perlop.

           In a "while (<>)" loop, "eof" or "eof(ARGV)" can be used to detect the end of each
           file, whereas "eof()" will detect the end of the very last file only.  Examples:

               # reset line numbering on each input file
               while (<>) {
                   next if /^\s*#/;  # skip comments
                   print "$.\t$_";
               } continue {
                   close ARGV if eof;  # Not eof()!
               }

               # insert dashes just before last line of last file
               while (<>) {
                   if (eof()) {  # check for end of last file
                       print "--------------\n";
                   }
                   print;
                   last if eof();     # needed if we're reading from a terminal
               }

           Practical hint: you almost never need to use "eof" in Perl, because the input
           operators typically return "undef" when they run out of data or encounter an error.

       eval EXPR
       eval BLOCK
       eval
           "eval" in all its forms is used to execute a little Perl program, trapping any errors
           encountered so they don't crash the calling program.

           Plain "eval" with no argument is just "eval EXPR", where the expression is understood
           to be contained in $_.  Thus there are only two real "eval" forms; the one with an
           EXPR is often called "string eval".  In a string eval, the value of the expression
           (which is itself determined within scalar context) is first parsed, and if there were
           no errors, executed as a block within the lexical context of the current Perl program.
           This form is typically used to delay parsing and subsequent execution of the text of
           EXPR until run time.  Note that the value is parsed every time the "eval" executes.

           The other form is called "block eval".  It is less general than string eval, but the
           code within the BLOCK is parsed only once (at the same time the code surrounding the
           "eval" itself was parsed) and executed within the context of the current Perl program.
           This form is typically used to trap exceptions more efficiently than the first, while
           also providing the benefit of checking the code within BLOCK at compile time.  BLOCK
           is parsed and compiled just once.  Since errors are trapped, it often is used to check
           if a given feature is available.

           In both forms, the value returned is the value of the last expression evaluated inside
           the mini-program; a return statement may also be used, just as with subroutines.  The
           expression providing the return value is evaluated in void, scalar, or list context,
           depending on the context of the "eval" itself.  See "wantarray" for more on how the
           evaluation context can be determined.

           If there is a syntax error or runtime error, or a "die" statement is executed, "eval"
           returns "undef" in scalar context, or an empty list in list context, and $@ is set to
           the error message.  (Prior to 5.16, a bug caused "undef" to be returned in list
           context for syntax errors, but not for runtime errors.) If there was no error, $@ is
           set to the empty string.  A control flow operator like "last" or "goto" can bypass the
           setting of $@.  Beware that using "eval" neither silences Perl from printing warnings
           to STDERR, nor does it stuff the text of warning messages into $@.  To do either of
           those, you have to use the $SIG{__WARN__} facility, or turn off warnings inside the
           BLOCK or EXPR using "no warnings 'all'".  See "warn", perlvar, and warnings.

           Note that, because "eval" traps otherwise-fatal errors, it is useful for determining
           whether a particular feature (such as "socket" or "symlink") is implemented.  It is
           also Perl's exception-trapping mechanism, where the "die" operator is used to raise
           exceptions.

           Before Perl 5.14, the assignment to $@ occurred before restoration of localized
           variables, which means that for your code to run on older versions, a temporary is
           required if you want to mask some, but not all errors:

            # alter $@ on nefarious repugnancy only
            {
               my $e;
               {
                 local $@; # protect existing $@
                 eval { test_repugnancy() };
                 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
                 $@ =~ /nefarious/ and $e = $@;
               }
               die $e if defined $e
            }

           There are some different considerations for each form:

           String eval
               Since the return value of EXPR is executed as a block within the lexical context
               of the current Perl program, any outer lexical variables are visible to it, and
               any package variable settings or subroutine and format definitions remain
               afterwards.

               Under the "unicode_eval" feature
                   If this feature is enabled (which is the default under a "use 5.16" or higher
                   declaration), EXPR is considered to be in the same encoding as the surrounding
                   program.  Thus if "use utf8" is in effect, the string will be treated as being
                   UTF-8 encoded.  Otherwise, the string is considered to be a sequence of
                   independent bytes.  Bytes that correspond to ASCII-range code points will have
                   their normal meanings for operators in the string.  The treatment of the other
                   bytes depends on if the "'unicode_strings"" feature is in effect.

                   In a plain "eval" without an EXPR argument, being in "use utf8" or not is
                   irrelevant; the UTF-8ness of $_ itself determines the behavior.

                   Any "use utf8" or "no utf8" declarations within the string have no effect, and
                   source filters are forbidden.  ("unicode_strings", however, can appear within
                   the string.)  See also the "evalbytes" operator, which works properly with
                   source filters.

                   Variables defined outside the "eval" and used inside it retain their original
                   UTF-8ness.  Everything inside the string follows the normal rules for a Perl
                   program with the given state of "use utf8".

               Outside the "unicode_eval" feature
                   In this case, the behavior is problematic and is not so easily described.
                   Here are two bugs that cannot easily be fixed without breaking existing
                   programs:

                   ·   It can lose track of whether something should be encoded as UTF-8 or not.

                   ·   Source filters activated within "eval" leak out into whichever file scope
                       is currently being compiled.  To give an example with the CPAN module
                       Semi::Semicolons:

                        BEGIN { eval "use Semi::Semicolons; # not filtered" }
                        # filtered here!

                       "evalbytes" fixes that to work the way one would expect:

                        use feature "evalbytes";
                        BEGIN { evalbytes "use Semi::Semicolons; # filtered" }
                        # not filtered

               Problems can arise if the string expands a scalar containing a floating point
               number.  That scalar can expand to letters, such as "NaN" or "Infinity"; or,
               within the scope of a "use locale", the decimal point character may be something
               other than a dot (such as a comma).  None of these are likely to parse as you are
               likely expecting.

               You should be especially careful to remember what's being looked at when:

                   eval $x;        # CASE 1
                   eval "$x";      # CASE 2

                   eval '$x';      # CASE 3
                   eval { $x };    # CASE 4

                   eval "\$$x++";  # CASE 5
                   $$x++;          # CASE 6

               Cases 1 and 2 above behave identically: they run the code contained in the
               variable $x.  (Although case 2 has misleading double quotes making the reader
               wonder what else might be happening (nothing is).)  Cases 3 and 4 likewise behave
               in the same way: they run the code '$x', which does nothing but return the value
               of $x.  (Case 4 is preferred for purely visual reasons, but it also has the
               advantage of compiling at compile-time instead of at run-time.)  Case 5 is a place
               where normally you would like to use double quotes, except that in this particular
               situation, you can just use symbolic references instead, as in case 6.

               An "eval ''" executed within a subroutine defined in the "DB" package doesn't see
               the usual surrounding lexical scope, but rather the scope of the first non-DB
               piece of code that called it.  You don't normally need to worry about this unless
               you are writing a Perl debugger.

               The final semicolon, if any, may be omitted from the value of EXPR.

           Block eval
               If the code to be executed doesn't vary, you may use the eval-BLOCK form to trap
               run-time errors without incurring the penalty of recompiling each time.  The
               error, if any, is still returned in $@.  Examples:

                   # make divide-by-zero nonfatal
                   eval { $answer = $a / $b; }; warn $@ if $@;

                   # same thing, but less efficient
                   eval '$answer = $a / $b'; warn $@ if $@;

                   # a compile-time error
                   eval { $answer = }; # WRONG

                   # a run-time error
                   eval '$answer =';   # sets $@

               If you want to trap errors when loading an XS module, some problems with the
               binary interface (such as Perl version skew) may be fatal even with "eval" unless
               $ENV{PERL_DL_NONLAZY} is set.  See perlrun.

               Using the "eval {}" form as an exception trap in libraries does have some issues.
               Due to the current arguably broken state of "__DIE__" hooks, you may wish not to
               trigger any "__DIE__" hooks that user code may have installed.  You can use the
               "local $SIG{__DIE__}" construct for this purpose, as this example shows:

                   # a private exception trap for divide-by-zero
                   eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
                   warn $@ if $@;

               This is especially significant, given that "__DIE__" hooks can call "die" again,
               which has the effect of changing their error messages:

                   # __DIE__ hooks may modify error messages
                   {
                      local $SIG{'__DIE__'} =
                             sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
                      eval { die "foo lives here" };
                      print $@ if $@;                # prints "bar lives here"
                   }

               Because this promotes action at a distance, this counterintuitive behavior may be
               fixed in a future release.

               "eval BLOCK" does not count as a loop, so the loop control statements "next",
               "last", or "redo" cannot be used to leave or restart the block.

               The final semicolon, if any, may be omitted from within the BLOCK.

       evalbytes EXPR
       evalbytes
           This function is similar to a string eval, except it always parses its argument (or $_
           if EXPR is omitted) as a string of independent bytes.

           If called when "use utf8" is in effect, the string will be assumed to be encoded in
           UTF-8, and "evalbytes" will make a temporary copy to work from, downgraded to
           non-UTF-8.  If this is not possible (because one or more characters in it require
           UTF-8), the "evalbytes" will fail with the error stored in $@.

           Bytes that correspond to ASCII-range code points will have their normal meanings for
           operators in the string.  The treatment of the other bytes depends on if the
           "'unicode_strings"" feature is in effect.

           Of course, variables that are UTF-8 and are referred to in the string retain that:

            my $a = "\x{100}";
            evalbytes 'print ord $a, "\n"';

           prints

            256

           and $@ is empty.

           Source filters activated within the evaluated code apply to the code itself.

           "evalbytes" is available starting in Perl v5.16.  To access it, you must say
           "CORE::evalbytes", but you can omit the "CORE::" if the "evalbytes" feature is
           enabled.  This is enabled automatically with a "use v5.16" (or higher) declaration in
           the current scope.

       exec LIST
       exec PROGRAM LIST
           The "exec" function executes a system command and never returns; use "system" instead
           of "exec" if you want it to return.  It fails and returns false only if the command
           does not exist and it is executed directly instead of via your system's command shell
           (see below).

           Since it's a common mistake to use "exec" instead of "system", Perl warns you if
           "exec" is called in void context and if there is a following statement that isn't
           "die", "warn", or "exit" (if warnings are enabled--but you always do that, right?).
           If you really want to follow an "exec" with some other statement, you can use one of
           these styles to avoid the warning:

               exec ('foo')   or print STDERR "couldn't exec foo: $!";
               { exec ('foo') }; print STDERR "couldn't exec foo: $!";

           If there is more than one argument in LIST, this calls execvp(3) with the arguments in
           LIST.  If there is only one element in LIST, the argument is checked for shell
           metacharacters, and if there are any, the entire argument is passed to the system's
           command shell for parsing (this is "/bin/sh -c" on Unix platforms, but varies on other
           platforms).  If there are no shell metacharacters in the argument, it is split into
           words and passed directly to "execvp", which is more efficient.  Examples:

               exec '/bin/echo', 'Your arguments are: ', @ARGV;
               exec "sort $outfile | uniq";

           If you don't really want to execute the first argument, but want to lie to the program
           you are executing about its own name, you can specify the program you actually want to
           run as an "indirect object" (without a comma) in front of the LIST, as in "exec
           PROGRAM LIST".  (This always forces interpretation of the LIST as a multivalued list,
           even if there is only a single scalar in the list.)  Example:

               my $shell = '/bin/csh';
               exec $shell '-sh';    # pretend it's a login shell

           or, more directly,

               exec {'/bin/csh'} '-sh';  # pretend it's a login shell

           When the arguments get executed via the system shell, results are subject to its
           quirks and capabilities.  See "`STRING`" in perlop for details.

           Using an indirect object with "exec" or "system" is also more secure.  This usage
           (which also works fine with "system") forces interpretation of the arguments as a
           multivalued list, even if the list had just one argument.  That way you're safe from
           the shell expanding wildcards or splitting up words with whitespace in them.

               my @args = ( "echo surprise" );

               exec @args;               # subject to shell escapes
                                           # if @args == 1
               exec { $args[0] } @args;  # safe even with one-arg list

           The first version, the one without the indirect object, ran the echo program, passing
           it "surprise" an argument.  The second version didn't; it tried to run a program named
           "echo surprise", didn't find it, and set $? to a non-zero value indicating failure.

           On Windows, only the "exec PROGRAM LIST" indirect object syntax will reliably avoid
           using the shell; "exec LIST", even with more than one element, will fall back to the
           shell if the first spawn fails.

           Perl attempts to flush all files opened for output before the exec, but this may not
           be supported on some platforms (see perlport).  To be safe, you may need to set $|
           ($AUTOFLUSH in English) or call the "autoflush" method of "IO::Handle" on any open
           handles to avoid lost output.

           Note that "exec" will not call your "END" blocks, nor will it invoke "DESTROY" methods
           on your objects.

           Portability issues: "exec" in perlport.

       exists EXPR
           Given an expression that specifies an element of a hash, returns true if the specified
           element in the hash has ever been initialized, even if the corresponding value is
           undefined.

               print "Exists\n"    if exists $hash{$key};
               print "Defined\n"   if defined $hash{$key};
               print "True\n"      if $hash{$key};

           exists may also be called on array elements, but its behavior is much less obvious and
           is strongly tied to the use of "delete" on arrays.

           WARNING: Calling "exists" on array values is strongly discouraged.  The notion of
           deleting or checking the existence of Perl array elements is not conceptually
           coherent, and can lead to surprising behavior.

               print "Exists\n"    if exists $array[$index];
               print "Defined\n"   if defined $array[$index];
               print "True\n"      if $array[$index];

           A hash or array element can be true only if it's defined and defined only if it
           exists, but the reverse doesn't necessarily hold true.

           Given an expression that specifies the name of a subroutine, returns true if the
           specified subroutine has ever been declared, even if it is undefined.  Mentioning a
           subroutine name for exists or defined does not count as declaring it.  Note that a
           subroutine that does not exist may still be callable: its package may have an
           "AUTOLOAD" method that makes it spring into existence the first time that it is
           called; see perlsub.

               print "Exists\n"  if exists &subroutine;
               print "Defined\n" if defined &subroutine;

           Note that the EXPR can be arbitrarily complicated as long as the final operation is a
           hash or array key lookup or subroutine name:

               if (exists $ref->{A}->{B}->{$key})  { }
               if (exists $hash{A}{B}{$key})       { }

               if (exists $ref->{A}->{B}->[$ix])   { }
               if (exists $hash{A}{B}[$ix])        { }

               if (exists &{$ref->{A}{B}{$key}})   { }

           Although the most deeply nested array or hash element will not spring into existence
           just because its existence was tested, any intervening ones will.  Thus "$ref->{"A"}"
           and "$ref->{"A"}->{"B"}" will spring into existence due to the existence test for the
           $key element above.  This happens anywhere the arrow operator is used, including even
           here:

               undef $ref;
               if (exists $ref->{"Some key"})    { }
               print $ref;  # prints HASH(0x80d3d5c)

           This surprising autovivification in what does not at first--or even second--glance
           appear to be an lvalue context may be fixed in a future release.

           Use of a subroutine call, rather than a subroutine name, as an argument to "exists" is
           an error.

               exists &sub;    # OK
               exists &sub();  # Error

       exit EXPR
       exit
           Evaluates EXPR and exits immediately with that value.    Example:

               my $ans = <STDIN>;
               exit 0 if $ans =~ /^[Xx]/;

           See also "die".  If EXPR is omitted, exits with 0 status.  The only universally
           recognized values for EXPR are 0 for success and 1 for error; other values are subject
           to interpretation depending on the environment in which the Perl program is running.
           For example, exiting 69 (EX_UNAVAILABLE) from a sendmail incoming-mail filter will
           cause the mailer to return the item undelivered, but that's not true everywhere.

           Don't use "exit" to abort a subroutine if there's any chance that someone might want
           to trap whatever error happened.  Use "die" instead, which can be trapped by an
           "eval".

           The "exit" function does not always exit immediately.  It calls any defined "END"
           routines first, but these "END" routines may not themselves abort the exit.  Likewise
           any object destructors that need to be called are called before the real exit.  "END"
           routines and destructors can change the exit status by modifying $?.  If this is a
           problem, you can call "POSIX::_exit($status)" to avoid "END" and destructor
           processing.  See perlmod for details.

           Portability issues: "exit" in perlport.

       exp EXPR
       exp Returns e (the natural logarithm base) to the power of EXPR.  If EXPR is omitted,
           gives "exp($_)".

       fc EXPR
       fc  Returns the casefolded version of EXPR.  This is the internal function implementing
           the "\F" escape in double-quoted strings.

           Casefolding is the process of mapping strings to a form where case differences are
           erased; comparing two strings in their casefolded form is effectively a way of asking
           if two strings are equal, regardless of case.

           Roughly, if you ever found yourself writing this

               lc($this) eq lc($that)    # Wrong!
                   # or
               uc($this) eq uc($that)    # Also wrong!
                   # or
               $this =~ /^\Q$that\E\z/i  # Right!

           Now you can write

               fc($this) eq fc($that)

           And get the correct results.

           Perl only implements the full form of casefolding, but you can access the simple folds
           using "casefold()" in Unicode::UCD and "prop_invmap()" in Unicode::UCD.  For further
           information on casefolding, refer to the Unicode Standard, specifically sections 3.13
           "Default Case Operations", 4.2 "Case-Normative", and 5.18 "Case Mappings", available
           at <http://www.unicode.org/versions/latest/>, as well as the Case Charts available at
           <http://www.unicode.org/charts/case/>.

           If EXPR is omitted, uses $_.

           This function behaves the same way under various pragmas, such as within
           "use feature 'unicode_strings", as "lc" does, with the single exception of "fc" of
           LATIN CAPITAL LETTER SHARP S (U+1E9E) within the scope of "use locale".  The foldcase
           of this character would normally be "ss", but as explained in the "lc" section, case
           changes that cross the 255/256 boundary are problematic under locales, and are hence
           prohibited.  Therefore, this function under locale returns instead the string
           "\x{17F}\x{17F}", which is the LATIN SMALL LETTER LONG S.  Since that character itself
           folds to "s", the string of two of them together should be equivalent to a single
           U+1E9E when foldcased.

           While the Unicode Standard defines two additional forms of casefolding, one for Turkic
           languages and one that never maps one character into multiple characters, these are
           not provided by the Perl core.  However, the CPAN module "Unicode::Casing" may be used
           to provide an implementation.

           "fc" is available only if the "fc" feature is enabled or if it is prefixed with
           "CORE::".  The "fc" feature is enabled automatically with a "use v5.16" (or higher)
           declaration in the current scope.

       fcntl FILEHANDLE,FUNCTION,SCALAR
           Implements the fcntl(2) function.  You'll probably have to say

               use Fcntl;

           first to get the correct constant definitions.  Argument processing and value returned
           work just like "ioctl" below.  For example:

               use Fcntl;
               my $flags = fcntl($filehandle, F_GETFL, 0)
                   or die "Can't fcntl F_GETFL: $!";

           You don't have to check for "defined" on the return from "fcntl".  Like "ioctl", it
           maps a 0 return from the system call into "0 but true" in Perl.  This string is true
           in boolean context and 0 in numeric context.  It is also exempt from the normal
           "Argument "..." isn't numeric" warnings on improper numeric conversions.

           Note that "fcntl" raises an exception if used on a machine that doesn't implement
           fcntl(2).  See the Fcntl module or your fcntl(2) manpage to learn what functions are
           available on your system.

           Here's an example of setting a filehandle named $REMOTE to be non-blocking at the
           system level.  You'll have to negotiate $| on your own, though.

               use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);

               my $flags = fcntl($REMOTE, F_GETFL, 0)
                   or die "Can't get flags for the socket: $!\n";

               fcntl($REMOTE, F_SETFL, $flags | O_NONBLOCK)
                   or die "Can't set flags for the socket: $!\n";

           Portability issues: "fcntl" in perlport.

       __FILE__
           A special token that returns the name of the file in which it occurs.

       fileno FILEHANDLE
           Returns the file descriptor for a filehandle, or undefined if the filehandle is not
           open.  If there is no real file descriptor at the OS level, as can happen with
           filehandles connected to memory objects via "open" with a reference for the third
           argument, -1 is returned.

           This is mainly useful for constructing bitmaps for "select" and low-level POSIX tty-
           handling operations.  If FILEHANDLE is an expression, the value is taken as an
           indirect filehandle, generally its name.

           You can use this to find out whether two handles refer to the same underlying
           descriptor:

               if (fileno($this) != -1 && fileno($this) == fileno($that)) {
                   print "\$this and \$that are dups\n";
               } elsif (fileno($this) != -1 && fileno($that) != -1) {
                   print "\$this and \$that have different " .
                       "underlying file descriptors\n";
               } else {
                   print "At least one of \$this and \$that does " .
                       "not have a real file descriptor\n";
               }

           The behavior of "fileno" on a directory handle depends on the operating system.  On a
           system with dirfd(3) or similar, "fileno" on a directory handle returns the underlying
           file descriptor associated with the handle; on systems with no such support, it
           returns the undefined value, and sets $! (errno).

       flock FILEHANDLE,OPERATION
           Calls flock(2), or an emulation of it, on FILEHANDLE.  Returns true for success, false
           on failure.  Produces a fatal error if used on a machine that doesn't implement
           flock(2), fcntl(2) locking, or lockf(3).  "flock" is Perl's portable file-locking
           interface, although it locks entire files only, not records.

           Two potentially non-obvious but traditional "flock" semantics are that it waits
           indefinitely until the lock is granted, and that its locks are merely advisory.  Such
           discretionary locks are more flexible, but offer fewer guarantees.  This means that
           programs that do not also use "flock" may modify files locked with "flock".  See
           perlport, your port's specific documentation, and your system-specific local manpages
           for details.  It's best to assume traditional behavior if you're writing portable
           programs.  (But if you're not, you should as always feel perfectly free to write for
           your own system's idiosyncrasies (sometimes called "features").  Slavish adherence to
           portability concerns shouldn't get in the way of your getting your job done.)

           OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with LOCK_NB.
           These constants are traditionally valued 1, 2, 8 and 4, but you can use the symbolic
           names if you import them from the Fcntl module, either individually, or as a group
           using the ":flock" tag.  LOCK_SH requests a shared lock, LOCK_EX requests an exclusive
           lock, and LOCK_UN releases a previously requested lock.  If LOCK_NB is bitwise-or'ed
           with LOCK_SH or LOCK_EX, then "flock" returns immediately rather than blocking waiting
           for the lock; check the return status to see if you got it.

           To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE before
           locking or unlocking it.

           Note that the emulation built with lockf(3) doesn't provide shared locks, and it
           requires that FILEHANDLE be open with write intent.  These are the semantics that
           lockf(3) implements.  Most if not all systems implement lockf(3) in terms of fcntl(2)
           locking, though, so the differing semantics shouldn't bite too many people.

           Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE be open with
           read intent to use LOCK_SH and requires that it be open with write intent to use
           LOCK_EX.

           Note also that some versions of "flock" cannot lock things over the network; you would
           need to use the more system-specific "fcntl" for that.  If you like you can force Perl
           to ignore your system's flock(2) function, and so provide its own fcntl(2)-based
           emulation, by passing the switch "-Ud_flock" to the Configure program when you
           configure and build a new Perl.

           Here's a mailbox appender for BSD systems.

               # import LOCK_* and SEEK_END constants
               use Fcntl qw(:flock SEEK_END);

               sub lock {
                   my ($fh) = @_;
                   flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";

                   # and, in case someone appended while we were waiting...
                   seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
               }

               sub unlock {
                   my ($fh) = @_;
                   flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
               }

               open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
                   or die "Can't open mailbox: $!";

               lock($mbox);
               print $mbox $msg,"\n\n";
               unlock($mbox);

           On systems that support a real flock(2), locks are inherited across "fork" calls,
           whereas those that must resort to the more capricious fcntl(2) function lose their
           locks, making it seriously harder to write servers.

           See also DB_File for other "flock" examples.

           Portability issues: "flock" in perlport.

       fork
           Does a fork(2) system call to create a new process running the same program at the
           same point.  It returns the child pid to the parent process, 0 to the child process,
           or "undef" if the fork is unsuccessful.  File descriptors (and sometimes locks on
           those descriptors) are shared, while everything else is copied.  On most systems
           supporting fork(2), great care has gone into making it extremely efficient (for
           example, using copy-on-write technology on data pages), making it the dominant
           paradigm for multitasking over the last few decades.

           Perl attempts to flush all files opened for output before forking the child process,
           but this may not be supported on some platforms (see perlport).  To be safe, you may
           need to set $| ($AUTOFLUSH in English) or call the "autoflush" method of "IO::Handle"
           on any open handles to avoid duplicate output.

           If you "fork" without ever waiting on your children, you will accumulate zombies.  On
           some systems, you can avoid this by setting $SIG{CHLD} to "IGNORE".  See also perlipc
           for more examples of forking and reaping moribund children.

           Note that if your forked child inherits system file descriptors like STDIN and STDOUT
           that are actually connected by a pipe or socket, even if you exit, then the remote
           server (such as, say, a CGI script or a backgrounded job launched from a remote shell)
           won't think you're done.  You should reopen those to /dev/null if it's any issue.

           On some platforms such as Windows, where the fork(2) system call is not available,
           Perl can be built to emulate "fork" in the Perl interpreter.  The emulation is
           designed, at the level of the Perl program, to be as compatible as possible with the
           "Unix" fork(2).  However it has limitations that have to be considered in code
           intended to be portable.  See perlfork for more details.

           Portability issues: "fork" in perlport.

       format
           Declare a picture format for use by the "write" function.  For example:

               format Something =
                   Test: @<<<<<<<< @||||| @>>>>>
                         $str,     $%,    '$' . int($num)
               .

               $str = "widget";
               $num = $cost/$quantity;
               $~ = 'Something';
               write;

           See perlform for many details and examples.

       formline PICTURE,LIST
           This is an internal function used by "format"s, though you may call it, too.  It
           formats (see perlform) a list of values according to the contents of PICTURE, placing
           the output into the format output accumulator, $^A (or $ACCUMULATOR in English).
           Eventually, when a "write" is done, the contents of $^A are written to some
           filehandle.  You could also read $^A and then set $^A back to "".  Note that a format
           typically does one "formline" per line of form, but the "formline" function itself
           doesn't care how many newlines are embedded in the PICTURE.  This means that the "~"
           and "~~" tokens treat the entire PICTURE as a single line.  You may therefore need to
           use multiple formlines to implement a single record format, just like the "format"
           compiler.

           Be careful if you put double quotes around the picture, because an "@" character may
           be taken to mean the beginning of an array name.  "formline" always returns true.  See
           perlform for other examples.

           If you are trying to use this instead of "write" to capture the output, you may find
           it easier to open a filehandle to a scalar ("open my $fh, ">", \$output") and write to
           that instead.

       getc FILEHANDLE
       getc
           Returns the next character from the input file attached to FILEHANDLE, or the
           undefined value at end of file or if there was an error (in the latter case $! is
           set).  If FILEHANDLE is omitted, reads from STDIN.  This is not particularly
           efficient.  However, it cannot be used by itself to fetch single characters without
           waiting for the user to hit enter.  For that, try something more like:

               if ($BSD_STYLE) {
                   system "stty cbreak </dev/tty >/dev/tty 2>&1";
               }
               else {
                   system "stty", '-icanon', 'eol', "\001";
               }

               my $key = getc(STDIN);

               if ($BSD_STYLE) {
                   system "stty -cbreak </dev/tty >/dev/tty 2>&1";
               }
               else {
                   system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
               }
               print "\n";

           Determination of whether $BSD_STYLE should be set is left as an exercise to the
           reader.

           The "POSIX::getattr" function can do this more portably on systems purporting POSIX
           compliance.  See also the "Term::ReadKey" module on CPAN.

       getlogin
           This implements the C library function of the same name, which on most systems returns
           the current login from /etc/utmp, if any.  If it returns the empty string, use
           "getpwuid".

               my $login = getlogin || getpwuid($<) || "Kilroy";

           Do not consider "getlogin" for authentication: it is not as secure as "getpwuid".

           Portability issues: "getlogin" in perlport.

       getpeername SOCKET
           Returns the packed sockaddr address of the other end of the SOCKET connection.

               use Socket;
               my $hersockaddr    = getpeername($sock);
               my ($port, $iaddr) = sockaddr_in($hersockaddr);
               my $herhostname    = gethostbyaddr($iaddr, AF_INET);
               my $herstraddr     = inet_ntoa($iaddr);

       getpgrp PID
           Returns the current process group for the specified PID.  Use a PID of 0 to get the
           current process group for the current process.  Will raise an exception if used on a
           machine that doesn't implement getpgrp(2).  If PID is omitted, returns the process
           group of the current process.  Note that the POSIX version of "getpgrp" does not
           accept a PID argument, so only "PID==0" is truly portable.

           Portability issues: "getpgrp" in perlport.

       getppid
           Returns the process id of the parent process.

           Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work around non-POSIX
           thread semantics the minority of Linux systems (and Debian GNU/kFreeBSD systems) that
           used LinuxThreads, this emulation has since been removed.  See the documentation for
           $$ for details.

           Portability issues: "getppid" in perlport.

       getpriority WHICH,WHO
           Returns the current priority for a process, a process group, or a user.  (See
           getpriority(2).)  Will raise a fatal exception if used on a machine that doesn't
           implement getpriority(2).

           Portability issues: "getpriority" in perlport.

       getpwnam NAME
       getgrnam NAME
       gethostbyname NAME
       getnetbyname NAME
       getprotobyname NAME
       getpwuid UID
       getgrgid GID
       getservbyname NAME,PROTO
       gethostbyaddr ADDR,ADDRTYPE
       getnetbyaddr ADDR,ADDRTYPE
       getprotobynumber NUMBER
       getservbyport PORT,PROTO
       getpwent
       getgrent
       gethostent
       getnetent
       getprotoent
       getservent
       setpwent
       setgrent
       sethostent STAYOPEN
       setnetent STAYOPEN
       setprotoent STAYOPEN
       setservent STAYOPEN
       endpwent
       endgrent
       endhostent
       endnetent
       endprotoent
       endservent
           These routines are the same as their counterparts in the system C library.  In list
           context, the return values from the various get routines are as follows:

            #    0        1          2           3         4
            my ( $name,   $passwd,   $gid,       $members  ) = getgr*
            my ( $name,   $aliases,  $addrtype,  $net      ) = getnet*
            my ( $name,   $aliases,  $port,      $proto    ) = getserv*
            my ( $name,   $aliases,  $proto                ) = getproto*
            my ( $name,   $aliases,  $addrtype,  $length,  @addrs ) = gethost*
            my ( $name,   $passwd,   $uid,       $gid,     $quota,
               $comment,  $gcos,     $dir,       $shell,   $expire ) = getpw*
            #    5        6          7           8         9

           (If the entry doesn't exist, the return value is a single meaningless true value.)

           The exact meaning of the $gcos field varies but it usually contains the real name of
           the user (as opposed to the login name) and other information pertaining to the user.
           Beware, however, that in many system users are able to change this information and
           therefore it cannot be trusted and therefore the $gcos is tainted (see perlsec).  The
           $passwd and $shell, user's encrypted password and login shell, are also tainted, for
           the same reason.

           In scalar context, you get the name, unless the function was a lookup by name, in
           which case you get the other thing, whatever it is.  (If the entry doesn't exist you
           get the undefined value.)  For example:

               my $uid   = getpwnam($name);
               my $name  = getpwuid($num);
               my $name  = getpwent();
               my $gid   = getgrnam($name);
               my $name  = getgrgid($num);
               my $name  = getgrent();
               # etc.

           In getpw*() the fields $quota, $comment, and $expire are special in that they are
           unsupported on many systems.  If the $quota is unsupported, it is an empty scalar.  If
           it is supported, it usually encodes the disk quota.  If the $comment field is
           unsupported, it is an empty scalar.  If it is supported it usually encodes some
           administrative comment about the user.  In some systems the $quota field may be
           $change or $age, fields that have to do with password aging.  In some systems the
           $comment field may be $class.  The $expire field, if present, encodes the expiration
           period of the account or the password.  For the availability and the exact meaning of
           these fields in your system, please consult getpwnam(3) and your system's pwd.h file.
           You can also find out from within Perl what your $quota and $comment fields mean and
           whether you have the $expire field by using the "Config" module and the values
           "d_pwquota", "d_pwage", "d_pwchange", "d_pwcomment", and "d_pwexpire".  Shadow
           password files are supported only if your vendor has implemented them in the intuitive
           fashion that calling the regular C library routines gets the shadow versions if you're
           running under privilege or if there exists the shadow(3) functions as found in System
           V (this includes Solaris and Linux).  Those systems that implement a proprietary
           shadow password facility are unlikely to be supported.

           The $members value returned by getgr*() is a space-separated list of the login names
           of the members of the group.

           For the gethost*() functions, if the "h_errno" variable is supported in C, it will be
           returned to you via $? if the function call fails.  The @addrs value returned by a
           successful call is a list of raw addresses returned by the corresponding library call.
           In the Internet domain, each address is four bytes long; you can unpack it by saying
           something like:

               my ($w,$x,$y,$z) = unpack('W4',$addr[0]);

           The Socket library makes this slightly easier:

               use Socket;
               my $iaddr = inet_aton("127.1"); # or whatever address
               my $name  = gethostbyaddr($iaddr, AF_INET);

               # or going the other way
               my $straddr = inet_ntoa($iaddr);

           In the opposite way, to resolve a hostname to the IP address you can write this:

               use Socket;
               my $packed_ip = gethostbyname("www.perl.org");
               my $ip_address;
               if (defined $packed_ip) {
                   $ip_address = inet_ntoa($packed_ip);
               }

           Make sure "gethostbyname" is called in SCALAR context and that its return value is
           checked for definedness.

           The "getprotobynumber" function, even though it only takes one argument, has the
           precedence of a list operator, so beware:

               getprotobynumber $number eq 'icmp'   # WRONG
               getprotobynumber($number eq 'icmp')  # actually means this
               getprotobynumber($number) eq 'icmp'  # better this way

           If you get tired of remembering which element of the return list contains which return
           value, by-name interfaces are provided in standard modules: "File::stat",
           "Net::hostent", "Net::netent", "Net::protoent", "Net::servent", "Time::gmtime",
           "Time::localtime", and "User::grent".  These override the normal built-ins, supplying
           versions that return objects with the appropriate names for each field.  For example:

              use File::stat;
              use User::pwent;
              my $is_his = (stat($filename)->uid == pwent($whoever)->uid);

           Even though it looks as though they're the same method calls (uid), they aren't,
           because a "File::stat" object is different from a "User::pwent" object.

           Portability issues: "getpwnam" in perlport to "endservent" in perlport.

       getsockname SOCKET
           Returns the packed sockaddr address of this end of the SOCKET connection, in case you
           don't know the address because you have several different IPs that the connection
           might have come in on.

               use Socket;
               my $mysockaddr = getsockname($sock);
               my ($port, $myaddr) = sockaddr_in($mysockaddr);
               printf "Connect to %s [%s]\n",
                  scalar gethostbyaddr($myaddr, AF_INET),
                  inet_ntoa($myaddr);

       getsockopt SOCKET,LEVEL,OPTNAME
           Queries the option named OPTNAME associated with SOCKET at a given LEVEL.  Options may
           exist at multiple protocol levels depending on the socket type, but at least the
           uppermost socket level SOL_SOCKET (defined in the "Socket" module) will exist.  To
           query options at another level the protocol number of the appropriate protocol
           controlling the option should be supplied.  For example, to indicate that an option is
           to be interpreted by the TCP protocol, LEVEL should be set to the protocol number of
           TCP, which you can get using "getprotobyname".

           The function returns a packed string representing the requested socket option, or
           "undef" on error, with the reason for the error placed in $!.  Just what is in the
           packed string depends on LEVEL and OPTNAME; consult getsockopt(2) for details.  A
           common case is that the option is an integer, in which case the result is a packed
           integer, which you can decode using "unpack" with the "i" (or "I") format.

           Here's an example to test whether Nagle's algorithm is enabled on a socket:

               use Socket qw(:all);

               defined(my $tcp = getprotobyname("tcp"))
                   or die "Could not determine the protocol number for tcp";
               # my $tcp = IPPROTO_TCP; # Alternative
               my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
                   or die "getsockopt TCP_NODELAY: $!";
               my $nodelay = unpack("I", $packed);
               print "Nagle's algorithm is turned ",
                      $nodelay ? "off\n" : "on\n";

           Portability issues: "getsockopt" in perlport.

       glob EXPR
       glob
           In list context, returns a (possibly empty) list of filename expansions on the value
           of EXPR such as the standard Unix shell /bin/csh would do.  In scalar context, glob
           iterates through such filename expansions, returning undef when the list is exhausted.
           This is the internal function implementing the "<*.c>" operator, but you can use it
           directly.  If EXPR is omitted, $_ is used.  The "<*.c>" operator is discussed in more
           detail in "I/O Operators" in perlop.

           Note that "glob" splits its arguments on whitespace and treats each segment as
           separate pattern.  As such, "glob("*.c *.h")" matches all files with a .c or .h
           extension.  The expression "glob(".* *")" matches all files in the current working
           directory.  If you want to glob filenames that might contain whitespace, you'll have
           to use extra quotes around the spacey filename to protect it.  For example, to glob
           filenames that have an "e" followed by a space followed by an "f", use one of:

               my @spacies = <"*e f*">;
               my @spacies = glob '"*e f*"';
               my @spacies = glob q("*e f*");

           If you had to get a variable through, you could do this:

               my @spacies = glob "'*${var}e f*'";
               my @spacies = glob qq("*${var}e f*");

           If non-empty braces are the only wildcard characters used in the "glob", no filenames
           are matched, but potentially many strings are returned.  For example, this produces
           nine strings, one for each pairing of fruits and colors:

               my @many = glob "{apple,tomato,cherry}={green,yellow,red}";

           This operator is implemented using the standard "File::Glob" extension.  See
           File::Glob for details, including "bsd_glob", which does not treat whitespace as a
           pattern separator.

           Portability issues: "glob" in perlport.

       gmtime EXPR
       gmtime
           Works just like "localtime" but the returned values are localized for the standard
           Greenwich time zone.

           Note: When called in list context, $isdst, the last value returned by gmtime, is
           always 0.  There is no Daylight Saving Time in GMT.

           Portability issues: "gmtime" in perlport.

       goto LABEL
       goto EXPR
       goto &NAME
           The "goto LABEL" form finds the statement labeled with LABEL and resumes execution
           there.  It can't be used to get out of a block or subroutine given to "sort".  It can
           be used to go almost anywhere else within the dynamic scope, including out of
           subroutines, but it's usually better to use some other construct such as "last" or
           "die".  The author of Perl has never felt the need to use this form of "goto" (in
           Perl, that is; C is another matter).  (The difference is that C does not offer named
           loops combined with loop control.  Perl does, and this replaces most structured uses
           of "goto" in other languages.)

           The "goto EXPR" form expects to evaluate "EXPR" to a code reference or a label name.
           If it evaluates to a code reference, it will be handled like "goto &NAME", below.
           This is especially useful for implementing tail recursion via "goto __SUB__".

           If the expression evaluates to a label name, its scope will be resolved dynamically.
           This allows for computed "goto"s per FORTRAN, but isn't necessarily recommended if
           you're optimizing for maintainability:

               goto ("FOO", "BAR", "GLARCH")[$i];

           As shown in this example, "goto EXPR" is exempt from the "looks like a function" rule.
           A pair of parentheses following it does not (necessarily) delimit its argument.
           "goto("NE")."XT"" is equivalent to "goto NEXT".  Also, unlike most named operators,
           this has the same precedence as assignment.

           Use of "goto LABEL" or "goto EXPR" to jump into a construct is deprecated and will
           issue a warning.  Even then, it may not be used to go into any construct that requires
           initialization, such as a subroutine or a "foreach" loop.  It also can't be used to go
           into a construct that is optimized away.

           The "goto &NAME" form is quite different from the other forms of "goto".  In fact, it
           isn't a goto in the normal sense at all, and doesn't have the stigma associated with
           other gotos.  Instead, it exits the current subroutine (losing any changes set by
           "local") and immediately calls in its place the named subroutine using the current
           value of @_.  This is used by "AUTOLOAD" subroutines that wish to load another
           subroutine and then pretend that the other subroutine had been called in the first
           place (except that any modifications to @_ in the current subroutine are propagated to
           the other subroutine.) After the "goto", not even "caller" will be able to tell that
           this routine was called first.

           NAME needn't be the name of a subroutine; it can be a scalar variable containing a
           code reference or a block that evaluates to a code reference.

       grep BLOCK LIST
       grep EXPR,LIST
           This is similar in spirit to, but not the same as, grep(1) and its relatives.  In
           particular, it is not limited to using regular expressions.

           Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
           element) and returns the list value consisting of those elements for which the
           expression evaluated to true.  In scalar context, returns the number of times the
           expression was true.

               my @foo = grep(!/^#/, @bar);    # weed out comments

           or equivalently,

               my @foo = grep {!/^#/} @bar;    # weed out comments

           Note that $_ is an alias to the list value, so it can be used to modify the elements
           of the LIST.  While this is useful and supported, it can cause bizarre results if the
           elements of LIST are not variables.  Similarly, grep returns aliases into the original
           list, much as a for loop's index variable aliases the list elements.  That is,
           modifying an element of a list returned by grep (for example, in a "foreach", "map" or
           another "grep") actually modifies the element in the original list.  This is usually
           something to be avoided when writing clear code.

           See also "map" for a list composed of the results of the BLOCK or EXPR.

       hex EXPR
       hex Interprets EXPR as a hex string and returns the corresponding numeric value.  If EXPR
           is omitted, uses $_.

               print hex '0xAf'; # prints '175'
               print hex 'aF';   # same
               $valid_input =~ /\A(?:0?[xX])?(?:_?[0-9a-fA-F])*\z/

           A hex string consists of hex digits and an optional "0x" or "x" prefix.  Each hex
           digit may be preceded by a single underscore, which will be ignored.  Any other
           character triggers a warning and causes the rest of the string to be ignored (even
           leading whitespace, unlike "oct").  Only integers can be represented, and integer
           overflow triggers a warning.

           To convert strings that might start with any of 0, "0x", or "0b", see "oct".  To
           present something as hex, look into "printf", "sprintf", and "unpack".

       import LIST
           There is no builtin "import" function.  It is just an ordinary method (subroutine)
           defined (or inherited) by modules that wish to export names to another module.  The
           "use" function calls the "import" method for the package used.  See also "use",
           perlmod, and Exporter.

       index STR,SUBSTR,POSITION
       index STR,SUBSTR
           The index function searches for one string within another, but without the wildcard-
           like behavior of a full regular-expression pattern match.  It returns the position of
           the first occurrence of SUBSTR in STR at or after POSITION.  If POSITION is omitted,
           starts searching from the beginning of the string.  POSITION before the beginning of
           the string or after its end is treated as if it were the beginning or the end,
           respectively.  POSITION and the return value are based at zero.  If the substring is
           not found, "index" returns -1.

       int EXPR
       int Returns the integer portion of EXPR.  If EXPR is omitted, uses $_.  You should not use
           this function for rounding: one because it truncates towards 0, and two because
           machine representations of floating-point numbers can sometimes produce
           counterintuitive results.  For example, "int(-6.725/0.025)" produces -268 rather than
           the correct -269; that's because it's really more like -268.99999999999994315658
           instead.  Usually, the "sprintf", "printf", or the "POSIX::floor" and "POSIX::ceil"
           functions will serve you better than will "int".

       ioctl FILEHANDLE,FUNCTION,SCALAR
           Implements the ioctl(2) function.  You'll probably first have to say

               require "sys/ioctl.ph";  # probably in
                                        # $Config{archlib}/sys/ioctl.ph

           to get the correct function definitions.  If sys/ioctl.ph doesn't exist or doesn't
           have the correct definitions you'll have to roll your own, based on your C header
           files such as <sys/ioctl.h>.  (There is a Perl script called h2ph that comes with the
           Perl kit that may help you in this, but it's nontrivial.)  SCALAR will be read and/or
           written depending on the FUNCTION; a C pointer to the string value of SCALAR will be
           passed as the third argument of the actual "ioctl" call.  (If SCALAR has no string
           value but does have a numeric value, that value will be passed rather than a pointer
           to the string value.  To guarantee this to be true, add a 0 to the scalar before using
           it.)  The "pack" and "unpack" functions may be needed to manipulate the values of
           structures used by "ioctl".

           The return value of "ioctl" (and "fcntl") is as follows:

               if OS returns:      then Perl returns:
                   -1               undefined value
                    0              string "0 but true"
               anything else           that number

           Thus Perl returns true on success and false on failure, yet you can still easily
           determine the actual value returned by the operating system:

               my $retval = ioctl(...) || -1;
               printf "System returned %d\n", $retval;

           The special string "0 but true" is exempt from "Argument "..." isn't numeric" warnings
           on improper numeric conversions.

           Portability issues: "ioctl" in perlport.

       join EXPR,LIST
           Joins the separate strings of LIST into a single string with fields separated by the
           value of EXPR, and returns that new string.  Example:

              my $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);

           Beware that unlike "split", "join" doesn't take a pattern as its first argument.
           Compare "split".

       keys HASH
       keys ARRAY
           Called in list context, returns a list consisting of all the keys of the named hash,
           or in Perl 5.12 or later only, the indices of an array.  Perl releases prior to 5.12
           will produce a syntax error if you try to use an array argument.  In scalar context,
           returns the number of keys or indices.

           Hash entries are returned in an apparently random order.  The actual random order is
           specific to a given hash; the exact same series of operations on two hashes may result
           in a different order for each hash.  Any insertion into the hash may change the order,
           as will any deletion, with the exception that the most recent key returned by "each"
           or "keys" may be deleted without changing the order.  So long as a given hash is
           unmodified you may rely on "keys", "values" and "each" to repeatedly return the same
           order as each other.  See "Algorithmic Complexity Attacks" in perlsec for details on
           why hash order is randomized.  Aside from the guarantees provided here the exact
           details of Perl's hash algorithm and the hash traversal order are subject to change in
           any release of Perl.  Tied hashes may behave differently to Perl's hashes with respect
           to changes in order on insertion and deletion of items.

           As a side effect, calling "keys" resets the internal iterator of the HASH or ARRAY
           (see "each").  In particular, calling "keys" in void context resets the iterator with
           no other overhead.

           Here is yet another way to print your environment:

               my @keys = keys %ENV;
               my @values = values %ENV;
               while (@keys) {
                   print pop(@keys), '=', pop(@values), "\n";
               }

           or how about sorted by key:

               foreach my $key (sort(keys %ENV)) {
                   print $key, '=', $ENV{$key}, "\n";
               }

           The returned values are copies of the original keys in the hash, so modifying them
           will not affect the original hash.  Compare "values".

           To sort a hash by value, you'll need to use a "sort" function.  Here's a descending
           numeric sort of a hash by its values:

               foreach my $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
                   printf "%4d %s\n", $hash{$key}, $key;
               }

           Used as an lvalue, "keys" allows you to increase the number of hash buckets allocated
           for the given hash.  This can gain you a measure of efficiency if you know the hash is
           going to get big.  (This is similar to pre-extending an array by assigning a larger
           number to $#array.)  If you say

               keys %hash = 200;

           then %hash will have at least 200 buckets allocated for it--256 of them, in fact,
           since it rounds up to the next power of two.  These buckets will be retained even if
           you do "%hash = ()", use "undef %hash" if you want to free the storage while %hash is
           still in scope.  You can't shrink the number of buckets allocated for the hash using
           "keys" in this way (but you needn't worry about doing this by accident, as trying has
           no effect).  "keys @array" in an lvalue context is a syntax error.

           Starting with Perl 5.14, an experimental feature allowed "keys" to take a scalar
           expression. This experiment has been deemed unsuccessful, and was removed as of Perl
           5.24.

           To avoid confusing would-be users of your code who are running earlier versions of
           Perl with mysterious syntax errors, put this sort of thing at the top of your file to
           signal that your code will work only on Perls of a recent vintage:

               use 5.012;  # so keys/values/each work on arrays

           See also "each", "values", and "sort".

       kill SIGNAL, LIST
       kill SIGNAL
           Sends a signal to a list of processes.  Returns the number of arguments that were
           successfully used to signal (which is not necessarily the same as the number of
           processes actually killed, e.g. where a process group is killed).

               my $cnt = kill 'HUP', $child1, $child2;
               kill 'KILL', @goners;

           SIGNAL may be either a signal name (a string) or a signal number.  A signal name may
           start with a "SIG" prefix, thus "FOO" and "SIGFOO" refer to the same signal.  The
           string form of SIGNAL is recommended for portability because the same signal may have
           different numbers in different operating systems.

           A list of signal names supported by the current platform can be found in
           $Config{sig_name}, which is provided by the "Config" module.  See Config for more
           details.

           A negative signal name is the same as a negative signal number, killing process groups
           instead of processes.  For example, "kill '-KILL', $pgrp" and "kill -9, $pgrp" will
           send "SIGKILL" to the entire process group specified.  That means you usually want to
           use positive not negative signals.

           If SIGNAL is either the number 0 or the string "ZERO" (or "SIGZERO"), no signal is
           sent to the process, but "kill" checks whether it's possible to send a signal to it
           (that means, to be brief, that the process is owned by the same user, or we are the
           super-user).  This is useful to check that a child process is still alive (even if
           only as a zombie) and hasn't changed its UID.  See perlport for notes on the
           portability of this construct.

           The behavior of kill when a PROCESS number is zero or negative depends on the
           operating system.  For example, on POSIX-conforming systems, zero will signal the
           current process group, -1 will signal all processes, and any other negative PROCESS
           number will act as a negative signal number and kill the entire process group
           specified.

           If both the SIGNAL and the PROCESS are negative, the results are undefined.  A warning
           may be produced in a future version.

           See "Signals" in perlipc for more details.

           On some platforms such as Windows where the fork(2) system call is not available, Perl
           can be built to emulate "fork" at the interpreter level.  This emulation has
           limitations related to kill that have to be considered, for code running on Windows
           and in code intended to be portable.

           See perlfork for more details.

           If there is no LIST of processes, no signal is sent, and the return value is 0.  This
           form is sometimes used, however, because it causes tainting checks to be run.  But see
           "Laundering and Detecting Tainted Data" in perlsec.

           Portability issues: "kill" in perlport.

       last LABEL
       last EXPR
       last
           The "last" command is like the "break" statement in C (as used in loops); it
           immediately exits the loop in question.  If the LABEL is omitted, the command refers
           to the innermost enclosing loop.  The "last EXPR" form, available starting in Perl
           5.18.0, allows a label name to be computed at run time, and is otherwise identical to
           "last LABEL".  The "continue" block, if any, is not executed:

               LINE: while (<STDIN>) {
                   last LINE if /^$/;  # exit when done with header
                   #...
               }

           "last" cannot be used to exit a block that returns a value such as "eval {}", "sub
           {}", or "do {}", and should not be used to exit a "grep" or "map" operation.

           Note that a block by itself is semantically identical to a loop that executes once.
           Thus "last" can be used to effect an early exit out of such a block.

           See also "continue" for an illustration of how "last", "next", and "redo" work.

           Unlike most named operators, this has the same precedence as assignment.  It is also
           exempt from the looks-like-a-function rule, so "last ("foo")."bar"" will cause "bar"
           to be part of the argument to "last".

       lc EXPR
       lc  Returns a lowercased version of EXPR.  This is the internal function implementing the
           "\L" escape in double-quoted strings.

           If EXPR is omitted, uses $_.

           What gets returned depends on several factors:

           If "use bytes" is in effect:
               The results follow ASCII rules.  Only the characters "A-Z" change, to "a-z"
               respectively.

           Otherwise, if "use locale" for "LC_CTYPE" is in effect:
               Respects current "LC_CTYPE" locale for code points < 256; and uses Unicode rules
               for the remaining code points (this last can only happen if the UTF8 flag is also
               set).  See perllocale.

               Starting in v5.20, Perl uses full Unicode rules if the locale is UTF-8.
               Otherwise, there is a deficiency in this scheme, which is that case changes that
               cross the 255/256 boundary are not well-defined.  For example, the lower case of
               LATIN CAPITAL LETTER SHARP S (U+1E9E) in Unicode rules is U+00DF (on ASCII
               platforms).   But under "use locale" (prior to v5.20 or not a UTF-8 locale), the
               lower case of U+1E9E is itself, because 0xDF may not be LATIN SMALL LETTER SHARP S
               in the current locale, and Perl has no way of knowing if that character even
               exists in the locale, much less what code point it is.  Perl returns a result that
               is above 255 (almost always the input character unchanged), for all instances (and
               there aren't many) where the 255/256 boundary would otherwise be crossed; and
               starting in v5.22, it raises a locale warning.

           Otherwise, If EXPR has the UTF8 flag set:
               Unicode rules are used for the case change.

           Otherwise, if "use feature 'unicode_strings'" or "use locale ':not_characters'" is in
           effect:
               Unicode rules are used for the case change.

           Otherwise:
               ASCII rules are used for the case change.  The lowercase of any character outside
               the ASCII range is the character itself.

       lcfirst EXPR
       lcfirst
           Returns the value of EXPR with the first character lowercased.  This is the internal
           function implementing the "\l" escape in double-quoted strings.

           If EXPR is omitted, uses $_.

           This function behaves the same way under various pragmas, such as in a locale, as "lc"
           does.

       length EXPR
       length
           Returns the length in characters of the value of EXPR.  If EXPR is omitted, returns
           the length of $_.  If EXPR is undefined, returns "undef".

           This function cannot be used on an entire array or hash to find out how many elements
           these have.  For that, use "scalar @array" and "scalar keys %hash", respectively.

           Like all Perl character operations, "length" normally deals in logical characters, not
           physical bytes.  For how many bytes a string encoded as UTF-8 would take up, use
           "length(Encode::encode('UTF-8', EXPR))" (you'll have to "use Encode" first).  See
           Encode and perlunicode.

       __LINE__
           A special token that compiles to the current line number.

       link OLDFILE,NEWFILE
           Creates a new filename linked to the old filename.  Returns true for success, false
           otherwise.

           Portability issues: "link" in perlport.

       listen SOCKET,QUEUESIZE
           Does the same thing that the listen(2) system call does.  Returns true if it
           succeeded, false otherwise.  See the example in "Sockets: Client/Server Communication"
           in perlipc.

       local EXPR
           You really probably want to be using "my" instead, because "local" isn't what most
           people think of as "local".  See "Private Variables via my()" in perlsub for details.

           A local modifies the listed variables to be local to the enclosing block, file, or
           eval.  If more than one value is listed, the list must be placed in parentheses.  See
           "Temporary Values via local()" in perlsub for details, including issues with tied
           arrays and hashes.

           The "delete local EXPR" construct can also be used to localize the deletion of
           array/hash elements to the current block.  See "Localized deletion of elements of
           composite types" in perlsub.

       localtime EXPR
       localtime
           Converts a time as returned by the time function to a 9-element list with the time
           analyzed for the local time zone.  Typically used as follows:

               #     0    1    2     3     4    5     6     7     8
               my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
                                                           localtime(time);

           All list elements are numeric and come straight out of the C `struct tm'.  $sec, $min,
           and $hour are the seconds, minutes, and hours of the specified time.

           $mday is the day of the month and $mon the month in the range 0..11, with 0 indicating
           January and 11 indicating December.  This makes it easy to get a month name from a
           list:

               my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
               print "$abbr[$mon] $mday";
               # $mon=9, $mday=18 gives "Oct 18"

           $year contains the number of years since 1900.  To get a 4-digit year write:

               $year += 1900;

           To get the last two digits of the year (e.g., "01" in 2001) do:

               $year = sprintf("%02d", $year % 100);

           $wday is the day of the week, with 0 indicating Sunday and 3 indicating Wednesday.
           $yday is the day of the year, in the range 0..364 (or 0..365 in leap years.)

           $isdst is true if the specified time occurs during Daylight Saving Time, false
           otherwise.

           If EXPR is omitted, "localtime" uses the current time (as returned by "time").

           In scalar context, "localtime" returns the ctime(3) value:

               my $now_string = localtime;  # e.g., "Thu Oct 13 04:54:34 1994"

           The format of this scalar value is not locale-dependent but built into Perl.  For GMT
           instead of local time use the "gmtime" builtin.  See also the "Time::Local" module
           (for converting seconds, minutes, hours, and such back to the integer value returned
           by "time"), and the POSIX module's "strftime" and "mktime" functions.

           To get somewhat similar but locale-dependent date strings, set up your locale
           environment variables appropriately (please see perllocale) and try for example:

               use POSIX qw(strftime);
               my $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
               # or for GMT formatted appropriately for your locale:
               my $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;

           Note that %a and %b, the short forms of the day of the week and the month of the year,
           may not necessarily be three characters wide.

           The Time::gmtime and Time::localtime modules provide a convenient, by-name access
           mechanism to the "gmtime" and "localtime" functions, respectively.

           For a comprehensive date and time representation look at the DateTime module on CPAN.

           Portability issues: "localtime" in perlport.

       lock THING
           This function places an advisory lock on a shared variable or referenced object
           contained in THING until the lock goes out of scope.

           The value returned is the scalar itself, if the argument is a scalar, or a reference,
           if the argument is a hash, array or subroutine.

           "lock" is a "weak keyword"; this means that if you've defined a function by this name
           (before any calls to it), that function will be called instead.  If you are not under
           "use threads::shared" this does nothing.  See threads::shared.

       log EXPR
       log Returns the natural logarithm (base e) of EXPR.  If EXPR is omitted, returns the log
           of $_.  To get the log of another base, use basic algebra: The base-N log of a number
           is equal to the natural log of that number divided by the natural log of N.  For
           example:

               sub log10 {
                   my $n = shift;
                   return log($n)/log(10);
               }

           See also "exp" for the inverse operation.

       lstat FILEHANDLE
       lstat EXPR
       lstat DIRHANDLE
       lstat
           Does the same thing as the "stat" function (including setting the special "_"
           filehandle) but stats a symbolic link instead of the file the symbolic link points to.
           If symbolic links are unimplemented on your system, a normal "stat" is done.  For much
           more detailed information, please see the documentation for "stat".

           If EXPR is omitted, stats $_.

           Portability issues: "lstat" in perlport.

       m// The match operator.  See "Regexp Quote-Like Operators" in perlop.

       map BLOCK LIST
       map EXPR,LIST
           Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
           element) and returns the list value composed of the results of each such evaluation.
           In scalar context, returns the total number of elements so generated.  Evaluates BLOCK
           or EXPR in list context, so each element of LIST may produce zero, one, or more
           elements in the returned value.

               my @chars = map(chr, @numbers);

           translates a list of numbers to the corresponding characters.

               my @squares = map { $_ * $_ } @numbers;

           translates a list of numbers to their squared values.

               my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;

           shows that number of returned elements can differ from the number of input elements.
           To omit an element, return an empty list ().  This could also be achieved by writing

               my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;

           which makes the intention more clear.

           Map always returns a list, which can be assigned to a hash such that the elements
           become key/value pairs.  See perldata for more details.

               my %hash = map { get_a_key_for($_) => $_ } @array;

           is just a funny way to write

               my %hash;
               foreach (@array) {
                   $hash{get_a_key_for($_)} = $_;
               }

           Note that $_ is an alias to the list value, so it can be used to modify the elements
           of the LIST.  While this is useful and supported, it can cause bizarre results if the
           elements of LIST are not variables.  Using a regular "foreach" loop for this purpose
           would be clearer in most cases.  See also "grep" for a list composed of those items of
           the original list for which the BLOCK or EXPR evaluates to true.

           "{" starts both hash references and blocks, so "map { ..." could be either the start
           of map BLOCK LIST or map EXPR, LIST.  Because Perl doesn't look ahead for the closing
           "}" it has to take a guess at which it's dealing with based on what it finds just
           after the "{".  Usually it gets it right, but if it doesn't it won't realize something
           is wrong until it gets to the "}" and encounters the missing (or unexpected) comma.
           The syntax error will be reported close to the "}", but you'll need to change
           something near the "{" such as using a unary "+" or semicolon to give Perl some help:

            my %hash = map {  "\L$_" => 1  } @array # perl guesses EXPR. wrong
            my %hash = map { +"\L$_" => 1  } @array # perl guesses BLOCK. right
            my %hash = map {; "\L$_" => 1  } @array # this also works
            my %hash = map { ("\L$_" => 1) } @array # as does this
            my %hash = map {  lc($_) => 1  } @array # and this.
            my %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!

            my %hash = map  ( lc($_), 1 ),   @array # evaluates to (1, @array)

           or to force an anon hash constructor use "+{":

               my @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
                                                         # comma at end

           to get a list of anonymous hashes each with only one entry apiece.

       mkdir FILENAME,MASK
       mkdir FILENAME
       mkdir
           Creates the directory specified by FILENAME, with permissions specified by MASK (as
           modified by "umask").  If it succeeds it returns true; otherwise it returns false and
           sets $! (errno).  MASK defaults to 0777 if omitted, and FILENAME defaults to $_ if
           omitted.

           In general, it is better to create directories with a permissive MASK and let the user
           modify that with their "umask" than it is to supply a restrictive MASK and give the
           user no way to be more permissive.  The exceptions to this rule are when the file or
           directory should be kept private (mail files, for instance).  The documentation for
           "umask" discusses the choice of MASK in more detail.

           Note that according to the POSIX 1003.1-1996 the FILENAME may have any number of
           trailing slashes.  Some operating and filesystems do not get this right, so Perl
           automatically removes all trailing slashes to keep everyone happy.

           To recursively create a directory structure, look at the "make_path" function of the
           File::Path module.

       msgctl ID,CMD,ARG
           Calls the System V IPC function msgctl(2).  You'll probably have to say

               use IPC::SysV;

           first to get the correct constant definitions.  If CMD is "IPC_STAT", then ARG must be
           a variable that will hold the returned "msqid_ds" structure.  Returns like "ioctl":
           the undefined value for error, "0 but true" for zero, or the actual return value
           otherwise.  See also "SysV IPC" in perlipc and the documentation for "IPC::SysV" and
           "IPC::Semaphore".

           Portability issues: "msgctl" in perlport.

       msgget KEY,FLAGS
           Calls the System V IPC function msgget(2).  Returns the message queue id, or "undef"
           on error.  See also "SysV IPC" in perlipc and the documentation for "IPC::SysV" and
           "IPC::Msg".

           Portability issues: "msgget" in perlport.

       msgrcv ID,VAR,SIZE,TYPE,FLAGS
           Calls the System V IPC function msgrcv to receive a message from message queue ID into
           variable VAR with a maximum message size of SIZE.  Note that when a message is
           received, the message type as a native long integer will be the first thing in VAR,
           followed by the actual message.  This packing may be opened with "unpack("l! a*")".
           Taints the variable.  Returns true if successful, false on error.  See also "SysV IPC"
           in perlipc and the documentation for "IPC::SysV" and "IPC::Msg".

           Portability issues: "msgrcv" in perlport.

       msgsnd ID,MSG,FLAGS
           Calls the System V IPC function msgsnd to send the message MSG to the message queue
           ID.  MSG must begin with the native long integer message type, be followed by the
           length of the actual message, and then finally the message itself.  This kind of
           packing can be achieved with "pack("l! a*", $type, $message)".  Returns true if
           successful, false on error.  See also "SysV IPC" in perlipc and the documentation for
           "IPC::SysV" and "IPC::Msg".

           Portability issues: "msgsnd" in perlport.

       my VARLIST
       my TYPE VARLIST
       my VARLIST : ATTRS
       my TYPE VARLIST : ATTRS
           A "my" declares the listed variables to be local (lexically) to the enclosing block,
           file, or "eval".  If more than one variable is listed, the list must be placed in
           parentheses.

           The exact semantics and interface of TYPE and ATTRS are still evolving.  TYPE may be a
           bareword, a constant declared with "use constant", or "__PACKAGE__".  It is currently
           bound to the use of the fields pragma, and attributes are handled using the attributes
           pragma, or starting from Perl 5.8.0 also via the Attribute::Handlers module.  See
           "Private Variables via my()" in perlsub for details.

           Note that with a parenthesised list, "undef" can be used as a dummy placeholder, for
           example to skip assignment of initial values:

               my ( undef, $min, $hour ) = localtime;

       next LABEL
       next EXPR
       next
           The "next" command is like the "continue" statement in C; it starts the next iteration
           of the loop:

               LINE: while (<STDIN>) {
                   next LINE if /^#/;  # discard comments
                   #...
               }

           Note that if there were a "continue" block on the above, it would get executed even on
           discarded lines.  If LABEL is omitted, the command refers to the innermost enclosing
           loop.  The "next EXPR" form, available as of Perl 5.18.0, allows a label name to be
           computed at run time, being otherwise identical to "next LABEL".

           "next" cannot be used to exit a block which returns a value such as "eval {}", "sub
           {}", or "do {}", and should not be used to exit a "grep" or "map" operation.

           Note that a block by itself is semantically identical to a loop that executes once.
           Thus "next" will exit such a block early.

           See also "continue" for an illustration of how "last", "next", and "redo" work.

           Unlike most named operators, this has the same precedence as assignment.  It is also
           exempt from the looks-like-a-function rule, so "next ("foo")."bar"" will cause "bar"
           to be part of the argument to "next".

       no MODULE VERSION LIST
       no MODULE VERSION
       no MODULE LIST
       no MODULE
       no VERSION
           See the "use" function, of which "no" is the opposite.

       oct EXPR
       oct Interprets EXPR as an octal string and returns the corresponding value.  (If EXPR
           happens to start off with "0x", interprets it as a hex string.  If EXPR starts off
           with "0b", it is interpreted as a binary string.  Leading whitespace is ignored in all
           three cases.)  The following will handle decimal, binary, octal, and hex in standard
           Perl notation:

               $val = oct($val) if $val =~ /^0/;

           If EXPR is omitted, uses $_.   To go the other way (produce a number in octal), use
           "sprintf" or "printf":

               my $dec_perms = (stat("filename"))[2] & 07777;
               my $oct_perm_str = sprintf "%o", $perms;

           The "oct" function is commonly used when a string such as 644 needs to be converted
           into a file mode, for example.  Although Perl automatically converts strings into
           numbers as needed, this automatic conversion assumes base 10.

           Leading white space is ignored without warning, as too are any trailing non-digits,
           such as a decimal point ("oct" only handles non-negative integers, not negative
           integers or floating point).

       open FILEHANDLE,EXPR
       open FILEHANDLE,MODE,EXPR
       open FILEHANDLE,MODE,EXPR,LIST
       open FILEHANDLE,MODE,REFERENCE
       open FILEHANDLE
           Opens the file whose filename is given by EXPR, and associates it with FILEHANDLE.

           Simple examples to open a file for reading:

               open(my $fh, "<", "input.txt")
                   or die "Can't open < input.txt: $!";

           and for writing:

               open(my $fh, ">", "output.txt")
                   or die "Can't open > output.txt: $!";

           (The following is a comprehensive reference to "open": for a gentler introduction you
           may consider perlopentut.)

           If FILEHANDLE is an undefined scalar variable (or array or hash element), a new
           filehandle is autovivified, meaning that the variable is assigned a reference to a
           newly allocated anonymous filehandle.  Otherwise if FILEHANDLE is an expression, its
           value is the real filehandle.  (This is considered a symbolic reference, so "use
           strict "refs"" should not be in effect.)

           If three (or more) arguments are specified, the open mode (including optional
           encoding) in the second argument are distinct from the filename in the third.  If MODE
           is "<" or nothing, the file is opened for input.  If MODE is ">", the file is opened
           for output, with existing files first being truncated ("clobbered") and nonexisting
           files newly created.  If MODE is ">>", the file is opened for appending, again being
           created if necessary.

           You can put a "+" in front of the ">" or "<" to indicate that you want both read and
           write access to the file; thus "+<" is almost always preferred for read/write
           updates--the "+>" mode would clobber the file first.  You can't usually use either
           read-write mode for updating textfiles, since they have variable-length records.  See
           the -i switch in perlrun for a better approach.  The file is created with permissions
           of 0666 modified by the process's "umask" value.

           These various prefixes correspond to the fopen(3) modes of "r", "r+", "w", "w+", "a",
           and "a+".

           In the one- and two-argument forms of the call, the mode and filename should be
           concatenated (in that order), preferably separated by white space.  You can--but
           shouldn't--omit the mode in these forms when that mode is "<".  It is safe to use the
           two-argument form of "open" if the filename argument is a known literal.

           For three or more arguments if MODE is "|-", the filename is interpreted as a command
           to which output is to be piped, and if MODE is "-|", the filename is interpreted as a
           command that pipes output to us.  In the two-argument (and one-argument) form, one
           should replace dash ("-") with the command.  See "Using open() for IPC" in perlipc for
           more examples of this.  (You are not allowed to "open" to a command that pipes both in
           and out, but see IPC::Open2, IPC::Open3, and "Bidirectional Communication with Another
           Process" in perlipc for alternatives.)

           In the form of pipe opens taking three or more arguments, if LIST is specified (extra
           arguments after the command name) then LIST becomes arguments to the command invoked
           if the platform supports it.  The meaning of "open" with more than three arguments for
           non-pipe modes is not yet defined, but experimental "layers" may give extra LIST
           arguments meaning.

           In the two-argument (and one-argument) form, opening "<-" or "-" opens STDIN and
           opening ">-" opens STDOUT.

           You may (and usually should) use the three-argument form of open to specify I/O layers
           (sometimes referred to as "disciplines") to apply to the handle that affect how the
           input and output are processed (see open and PerlIO for more details).  For example:

             open(my $fh, "<:encoding(UTF-8)", $filename)
               || die "Can't open UTF-8 encoded $filename: $!";

           opens the UTF8-encoded file containing Unicode characters; see perluniintro.  Note
           that if layers are specified in the three-argument form, then default layers stored in
           ${^OPEN} (see perlvar; usually set by the open pragma or the switch "-CioD") are
           ignored.  Those layers will also be ignored if you specify a colon with no name
           following it.  In that case the default layer for the operating system (:raw on Unix,
           :crlf on Windows) is used.

           Open returns nonzero on success, the undefined value otherwise.  If the "open"
           involved a pipe, the return value happens to be the pid of the subprocess.

           On some systems (in general, DOS- and Windows-based systems) "binmode" is necessary
           when you're not working with a text file.  For the sake of portability it is a good
           idea always to use it when appropriate, and never to use it when it isn't appropriate.
           Also, people can set their I/O to be by default UTF8-encoded Unicode, not bytes.

           When opening a file, it's seldom a good idea to continue if the request failed, so
           "open" is frequently used with "die".  Even if "die" won't do what you want (say, in a
           CGI script, where you want to format a suitable error message (but there are modules
           that can help with that problem)) always check the return value from opening a file.

           The filehandle will be closed when its reference count reaches zero.  If it is a
           lexically scoped variable declared with "my", that usually means the end of the
           enclosing scope.  However, this automatic close does not check for errors, so it is
           better to explicitly close filehandles, especially those used for writing:

               close($handle)
                  || warn "close failed: $!";

           An older style is to use a bareword as the filehandle, as

               open(FH, "<", "input.txt")
                  or die "Can't open < input.txt: $!";

           Then you can use "FH" as the filehandle, in "close FH" and "<FH>" and so on.  Note
           that it's a global variable, so this form is not recommended in new code.

           As a shortcut a one-argument call takes the filename from the global scalar variable
           of the same name as the filehandle:

               $ARTICLE = 100;
               open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";

           Here $ARTICLE must be a global (package) scalar variable - not one declared with "my"
           or "state".

           As a special case the three-argument form with a read/write mode and the third
           argument being "undef":

               open(my $tmp, "+>", undef) or die ...

           opens a filehandle to a newly created empty anonymous temporary file.  (This happens
           under any mode, which makes "+>" the only useful and sensible mode to use.)  You will
           need to "seek" to do the reading.

           Perl is built using PerlIO by default.  Unless you've changed this (such as building
           Perl with "Configure -Uuseperlio"), you can open filehandles directly to Perl scalars
           via:

               open(my $fh, ">", \$variable) || ..

           To (re)open "STDOUT" or "STDERR" as an in-memory file, close it first:

               close STDOUT;
               open(STDOUT, ">", \$variable)
                   or die "Can't open STDOUT: $!";

           See perliol for detailed info on PerlIO.

           General examples:

            open(my $log, ">>", "/usr/spool/news/twitlog");
            # if the open fails, output is discarded

            open(my $dbase, "+<", "dbase.mine")      # open for update
                or die "Can't open 'dbase.mine' for update: $!";

            open(my $dbase, "+<dbase.mine")          # ditto
                or die "Can't open 'dbase.mine' for update: $!";

            open(my $article_fh, "-|", "caesar <$article")  # decrypt
                                                            # article
                or die "Can't start caesar: $!";

            open(my $article_fh, "caesar <$article |")      # ditto
                or die "Can't start caesar: $!";

            open(my $out_fh, "|-", "sort >Tmp$$")    # $$ is our process id
                or die "Can't start sort: $!";

            # in-memory files
            open(my $memory, ">", \$var)
                or die "Can't open memory file: $!";
            print $memory "foo!\n";              # output will appear in $var

           You may also, in the Bourne shell tradition, specify an EXPR beginning with ">&", in
           which case the rest of the string is interpreted as the name of a filehandle (or file
           descriptor, if numeric) to be duped (as in dup(2)) and opened.  You may use "&" after
           ">", ">>", "<", "+>", "+>>", and "+<".  The mode you specify should match the mode of
           the original filehandle.  (Duping a filehandle does not take into account any existing
           contents of IO buffers.)  If you use the three-argument form, then you can pass either
           a number, the name of a filehandle, or the normal "reference to a glob".

           Here is a script that saves, redirects, and restores "STDOUT" and "STDERR" using
           various methods:

               #!/usr/bin/perl
               open(my $oldout, ">&STDOUT")     or die "Can't dup STDOUT: $!";
               open(OLDERR,     ">&", \*STDERR) or die "Can't dup STDERR: $!";

               open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
               open(STDERR, ">&STDOUT")     or die "Can't dup STDOUT: $!";

               select STDERR; $| = 1;  # make unbuffered
               select STDOUT; $| = 1;  # make unbuffered

               print STDOUT "stdout 1\n";  # this works for
               print STDERR "stderr 1\n";  # subprocesses too

               open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
               open(STDERR, ">&OLDERR")    or die "Can't dup OLDERR: $!";

               print STDOUT "stdout 2\n";
               print STDERR "stderr 2\n";

           If you specify '<&=X', where "X" is a file descriptor number or a filehandle, then
           Perl will do an equivalent of C's fdopen(3) of that file descriptor (and not call
           dup(2)); this is more parsimonious of file descriptors.  For example:

               # open for input, reusing the fileno of $fd
               open(my $fh, "<&=", $fd)

           or

               open(my $fh, "<&=$fd")

           or

               # open for append, using the fileno of $oldfh
               open(my $fh, ">>&=", $oldfh)

           Being parsimonious on filehandles is also useful (besides being parsimonious) for
           example when something is dependent on file descriptors, like for example locking
           using "flock".  If you do just "open(my $A, ">>&", $B)", the filehandle $A will not
           have the same file descriptor as $B, and therefore "flock($A)" will not "flock($B)"
           nor vice versa.  But with "open(my $A, ">>&=", $B)", the filehandles will share the
           same underlying system file descriptor.

           Note that under Perls older than 5.8.0, Perl uses the standard C library's' fdopen(3)
           to implement the "=" functionality.  On many Unix systems, fdopen(3) fails when file
           descriptors exceed a certain value, typically 255.  For Perls 5.8.0 and later, PerlIO
           is (most often) the default.

           You can see whether your Perl was built with PerlIO by running "perl -V:useperlio".
           If it says 'define', you have PerlIO; otherwise you don't.

           If you open a pipe on the command "-" (that is, specify either "|-" or "-|" with the
           one- or two-argument forms of "open"), an implicit "fork" is done, so "open" returns
           twice: in the parent process it returns the pid of the child process, and in the child
           process it returns (a defined) 0.  Use "defined($pid)" or "//" to determine whether
           the open was successful.

           For example, use either

              my $child_pid = open(my $from_kid, "-|") // die "Can't fork: $!";

           or

              my $child_pid = open(my $to_kid,   "|-") // die "Can't fork: $!";

           followed by

               if ($child_pid) {
                   # am the parent:
                   # either write $to_kid or else read $from_kid
                   ...
                  waitpid $child_pid, 0;
               } else {
                   # am the child; use STDIN/STDOUT normally
                   ...
                   exit;
               }

           The filehandle behaves normally for the parent, but I/O to that filehandle is piped
           from/to the STDOUT/STDIN of the child process.  In the child process, the filehandle
           isn't opened--I/O happens from/to the new STDOUT/STDIN.  Typically this is used like
           the normal piped open when you want to exercise more control over just how the pipe
           command gets executed, such as when running setuid and you don't want to have to scan
           shell commands for metacharacters.

           The following blocks are more or less equivalent:

               open(my $fh, "|tr '[a-z]' '[A-Z]'");
               open(my $fh, "|-", "tr '[a-z]' '[A-Z]'");
               open(my $fh, "|-") || exec 'tr', '[a-z]', '[A-Z]';
               open(my $fh, "|-", "tr", '[a-z]', '[A-Z]');

               open(my $fh, "cat -n '$file'|");
               open(my $fh, "-|", "cat -n '$file'");
               open(my $fh, "-|") || exec "cat", "-n", $file;
               open(my $fh, "-|", "cat", "-n", $file);

           The last two examples in each block show the pipe as "list form", which is not yet
           supported on all platforms.  A good rule of thumb is that if your platform has a real
           "fork" (in other words, if your platform is Unix, including Linux and MacOS X), you
           can use the list form.  You would want to use the list form of the pipe so you can
           pass literal arguments to the command without risk of the shell interpreting any shell
           metacharacters in them.  However, this also bars you from opening pipes to commands
           that intentionally contain shell metacharacters, such as:

               open(my $fh, "|cat -n | expand -4 | lpr")
                   || die "Can't open pipeline to lpr: $!";

           See "Safe Pipe Opens" in perlipc for more examples of this.

           Perl will attempt to flush all files opened for output before any operation that may
           do a fork, but this may not be supported on some platforms (see perlport).  To be
           safe, you may need to set $| ($AUTOFLUSH in English) or call the "autoflush" method of
           "IO::Handle" on any open handles.

           On systems that support a close-on-exec flag on files, the flag will be set for the
           newly opened file descriptor as determined by the value of $^F.  See "$^F" in perlvar.

           Closing any piped filehandle causes the parent process to wait for the child to
           finish, then returns the status value in $? and "${^CHILD_ERROR_NATIVE}".

           The filename passed to the one- and two-argument forms of "open" will have leading and
           trailing whitespace deleted and normal redirection characters honored.  This property,
           known as "magic open", can often be used to good effect.  A user could specify a
           filename of "rsh cat file |", or you could change certain filenames as needed:

               $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
               open(my $fh, $filename) or die "Can't open $filename: $!";

           Use the three-argument form to open a file with arbitrary weird characters in it,

               open(my $fh, "<", $file)
                   || die "Can't open $file: $!";

           otherwise it's necessary to protect any leading and trailing whitespace:

               $file =~ s#^(\s)#./$1#;
               open(my $fh, "< $file\0")
                   || die "Can't open $file: $!";

           (this may not work on some bizarre filesystems).  One should conscientiously choose
           between the magic and three-argument form of "open":

               open(my $in, $ARGV[0]) || die "Can't open $ARGV[0]: $!";

           will allow the user to specify an argument of the form "rsh cat file |", but will not
           work on a filename that happens to have a trailing space, while

               open(my $in, "<", $ARGV[0])
                   || die "Can't open $ARGV[0]: $!";

           will have exactly the opposite restrictions. (However, some shells support the syntax
           "perl your_program.pl <( rsh cat file )", which produces a filename that can be opened
           normally.)

           If you want a "real" C open(2), then you should use the "sysopen" function, which
           involves no such magic (but uses different filemodes than Perl "open", which
           corresponds to C fopen(3)).  This is another way to protect your filenames from
           interpretation.  For example:

               use IO::Handle;
               sysopen(my $fh, $path, O_RDWR|O_CREAT|O_EXCL)
                   or die "Can't open $path: $!";
               $fh->autoflush(1);
               print $fh "stuff $$\n";
               seek($fh, 0, 0);
               print "File contains: ", readline($fh);

           See "seek" for some details about mixing reading and writing.

           Portability issues: "open" in perlport.

       opendir DIRHANDLE,EXPR
           Opens a directory named EXPR for processing by "readdir", "telldir", "seekdir",
           "rewinddir", and "closedir".  Returns true if successful.  DIRHANDLE may be an
           expression whose value can be used as an indirect dirhandle, usually the real
           dirhandle name.  If DIRHANDLE is an undefined scalar variable (or array or hash
           element), the variable is assigned a reference to a new anonymous dirhandle; that is,
           it's autovivified.  DIRHANDLEs have their own namespace separate from FILEHANDLEs.

           See the example at "readdir".

       ord EXPR
       ord Returns the numeric value of the first character of EXPR.  If EXPR is an empty string,
           returns 0.  If EXPR is omitted, uses $_.  (Note character, not byte.)

           For the reverse, see "chr".  See perlunicode for more about Unicode.

       our VARLIST
       our TYPE VARLIST
       our VARLIST : ATTRS
       our TYPE VARLIST : ATTRS
           "our" makes a lexical alias to a package (i.e. global) variable of the same name in
           the current package for use within the current lexical scope.

           "our" has the same scoping rules as "my" or "state", meaning that it is only valid
           within a lexical scope.  Unlike "my" and "state", which both declare new (lexical)
           variables, "our" only creates an alias to an existing variable: a package variable of
           the same name.

           This means that when "use strict 'vars'" is in effect, "our" lets you use a package
           variable without qualifying it with the package name, but only within the lexical
           scope of the "our" declaration.  This applies immediately--even within the same
           statement.

               package Foo;
               use strict;

               $Foo::foo = 23;

               {
                   our $foo;   # alias to $Foo::foo
                   print $foo; # prints 23
               }

               print $Foo::foo; # prints 23

               print $foo; # ERROR: requires explicit package name

           This works even if the package variable has not been used before, as package variables
           spring into existence when first used.

               package Foo;
               use strict;

               our $foo = 23;   # just like $Foo::foo = 23

               print $Foo::foo; # prints 23

           Because the variable becomes legal immediately under "use strict 'vars'", so long as
           there is no variable with that name is already in scope, you can then reference the
           package variable again even within the same statement.

               package Foo;
               use strict;

               my  $foo = $foo; # error, undeclared $foo on right-hand side
               our $foo = $foo; # no errors

           If more than one variable is listed, the list must be placed in parentheses.

               our($bar, $baz);

           An "our" declaration declares an alias for a package variable that will be visible
           across its entire lexical scope, even across package boundaries.  The package in which
           the variable is entered is determined at the point of the declaration, not at the
           point of use.  This means the following behavior holds:

               package Foo;
               our $bar;      # declares $Foo::bar for rest of lexical scope
               $bar = 20;

               package Bar;
               print $bar;    # prints 20, as it refers to $Foo::bar

           Multiple "our" declarations with the same name in the same lexical scope are allowed
           if they are in different packages.  If they happen to be in the same package, Perl
           will emit warnings if you have asked for them, just like multiple "my" declarations.
           Unlike a second "my" declaration, which will bind the name to a fresh variable, a
           second "our" declaration in the same package, in the same scope, is merely redundant.

               use warnings;
               package Foo;
               our $bar;      # declares $Foo::bar for rest of lexical scope
               $bar = 20;

               package Bar;
               our $bar = 30; # declares $Bar::bar for rest of lexical scope
               print $bar;    # prints 30

               our $bar;      # emits warning but has no other effect
               print $bar;    # still prints 30

           An "our" declaration may also have a list of attributes associated with it.

           The exact semantics and interface of TYPE and ATTRS are still evolving.  TYPE is
           currently bound to the use of the fields pragma, and attributes are handled using the
           attributes pragma, or, starting from Perl 5.8.0, also via the Attribute::Handlers
           module.  See "Private Variables via my()" in perlsub for details.

           Note that with a parenthesised list, "undef" can be used as a dummy placeholder, for
           example to skip assignment of initial values:

               our ( undef, $min, $hour ) = localtime;

           "our" differs from "use vars", which allows use of an unqualified name only within the
           affected package, but across scopes.

       pack TEMPLATE,LIST
           Takes a LIST of values and converts it into a string using the rules given by the
           TEMPLATE.  The resulting string is the concatenation of the converted values.
           Typically, each converted value looks like its machine-level representation.  For
           example, on 32-bit machines an integer may be represented by a sequence of 4 bytes,
           which  will in Perl be presented as a string that's 4 characters long.

           See perlpacktut for an introduction to this function.

           The TEMPLATE is a sequence of characters that give the order and type of values, as
           follows:

               a  A string with arbitrary binary data, will be null padded.
               A  A text (ASCII) string, will be space padded.
               Z  A null-terminated (ASCIZ) string, will be null padded.

               b  A bit string (ascending bit order inside each byte,
                  like vec()).
               B  A bit string (descending bit order inside each byte).
               h  A hex string (low nybble first).
               H  A hex string (high nybble first).

               c  A signed char (8-bit) value.
               C  An unsigned char (octet) value.
               W  An unsigned char value (can be greater than 255).

               s  A signed short (16-bit) value.
               S  An unsigned short value.

               l  A signed long (32-bit) value.
               L  An unsigned long value.

               q  A signed quad (64-bit) value.
               Q  An unsigned quad value.
                    (Quads are available only if your system supports 64-bit
                     integer values _and_ if Perl has been compiled to support
                     those.  Raises an exception otherwise.)

               i  A signed integer value.
               I  A unsigned integer value.
                    (This 'integer' is _at_least_ 32 bits wide.  Its exact
                     size depends on what a local C compiler calls 'int'.)

               n  An unsigned short (16-bit) in "network" (big-endian) order.
               N  An unsigned long (32-bit) in "network" (big-endian) order.
               v  An unsigned short (16-bit) in "VAX" (little-endian) order.
               V  An unsigned long (32-bit) in "VAX" (little-endian) order.

               j  A Perl internal signed integer value (IV).
               J  A Perl internal unsigned integer value (UV).

               f  A single-precision float in native format.
               d  A double-precision float in native format.

               F  A Perl internal floating-point value (NV) in native format
               D  A float of long-double precision in native format.
                    (Long doubles are available only if your system supports
                     long double values _and_ if Perl has been compiled to
                     support those.  Raises an exception otherwise.
                     Note that there are different long double formats.)

               p  A pointer to a null-terminated string.
               P  A pointer to a structure (fixed-length string).

               u  A uuencoded string.
               U  A Unicode character number.  Encodes to a character in char-
                  acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
                  byte mode.

               w  A BER compressed integer (not an ASN.1 BER, see perlpacktut
                  for details).  Its bytes represent an unsigned integer in
                  base 128, most significant digit first, with as few digits
                  as possible.  Bit eight (the high bit) is set on each byte
                  except the last.

               x  A null byte (a.k.a ASCII NUL, "\000", chr(0))
               X  Back up a byte.
               @  Null-fill or truncate to absolute position, counted from the
                  start of the innermost ()-group.
               .  Null-fill or truncate to absolute position specified by
                  the value.
               (  Start of a ()-group.

           One or more modifiers below may optionally follow certain letters in the TEMPLATE (the
           second column lists letters for which the modifier is valid):

               !   sSlLiI     Forces native (short, long, int) sizes instead
                              of fixed (16-/32-bit) sizes.

               !   xX         Make x and X act as alignment commands.

               !   nNvV       Treat integers as signed instead of unsigned.

               !   @.         Specify position as byte offset in the internal
                              representation of the packed string.  Efficient
                              but dangerous.

               >   sSiIlLqQ   Force big-endian byte-order on the type.
                   jJfFdDpP   (The "big end" touches the construct.)

               <   sSiIlLqQ   Force little-endian byte-order on the type.
                   jJfFdDpP   (The "little end" touches the construct.)

           The ">" and "<" modifiers can also be used on "()" groups to force a particular byte-
           order on all components in that group, including all its subgroups.

           The following rules apply:

           ·   Each letter may optionally be followed by a number indicating the repeat count.  A
               numeric repeat count may optionally be enclosed in brackets, as in "pack("C[80]",
               @arr)".  The repeat count gobbles that many values from the LIST when used with
               all format types other than "a", "A", "Z", "b", "B", "h", "H", "@", ".", "x", "X",
               and "P", where it means something else, described below.  Supplying a "*" for the
               repeat count instead of a number means to use however many items are left, except
               for:

               ·   "@", "x", and "X", where it is equivalent to 0.

               ·   <.>, where it means relative to the start of the string.

               ·   "u", where it is equivalent to 1 (or 45, which here is equivalent).

               One can replace a numeric repeat count with a template letter enclosed in brackets
               to use the packed byte length of the bracketed template for the repeat count.

               For example, the template "x[L]" skips as many bytes as in a packed long, and the
               template "$t X[$t] $t" unpacks twice whatever $t (when variable-expanded) unpacks.
               If the template in brackets contains alignment commands (such as "x![d]"), its
               packed length is calculated as if the start of the template had the maximal
               possible alignment.

               When used with "Z", a "*" as the repeat count is guaranteed to add a trailing null
               byte, so the resulting string is always one byte longer than the byte length of
               the item itself.

               When used with "@", the repeat count represents an offset from the start of the
               innermost "()" group.

               When used with ".", the repeat count determines the starting position to calculate
               the value offset as follows:

               ·   If the repeat count is 0, it's relative to the current position.

               ·   If the repeat count is "*", the offset is relative to the start of the packed
                   string.

               ·   And if it's an integer n, the offset is relative to the start of the nth
                   innermost "( )" group, or to the start of the string if n is bigger then the
                   group level.

               The repeat count for "u" is interpreted as the maximal number of bytes to encode
               per line of output, with 0, 1 and 2 replaced by 45.  The repeat count should not
               be more than 65.

           ·   The "a", "A", and "Z" types gobble just one value, but pack it as a string of
               length count, padding with nulls or spaces as needed.  When unpacking, "A" strips
               trailing whitespace and nulls, "Z" strips everything after the first null, and "a"
               returns data with no stripping at all.

               If the value to pack is too long, the result is truncated.  If it's too long and
               an explicit count is provided, "Z" packs only "$count-1" bytes, followed by a null
               byte.  Thus "Z" always packs a trailing null, except when the count is 0.

           ·   Likewise, the "b" and "B" formats pack a string that's that many bits long.  Each
               such format generates 1 bit of the result.  These are typically followed by a
               repeat count like "B8" or "B64".

               Each result bit is based on the least-significant bit of the corresponding input
               character, i.e., on "ord($char)%2".  In particular, characters "0" and "1"
               generate bits 0 and 1, as do characters "\000" and "\001".

               Starting from the beginning of the input string, each 8-tuple of characters is
               converted to 1 character of output.  With format "b", the first character of the
               8-tuple determines the least-significant bit of a character; with format "B", it
               determines the most-significant bit of a character.

               If the length of the input string is not evenly divisible by 8, the remainder is
               packed as if the input string were padded by null characters at the end.
               Similarly during unpacking, "extra" bits are ignored.

               If the input string is longer than needed, remaining characters are ignored.

               A "*" for the repeat count uses all characters of the input field.  On unpacking,
               bits are converted to a string of 0s and 1s.

           ·   The "h" and "H" formats pack a string that many nybbles (4-bit groups,
               representable as hexadecimal digits, "0".."9" "a".."f") long.

               For each such format, "pack" generates 4 bits of result.  With non-alphabetical
               characters, the result is based on the 4 least-significant bits of the input
               character, i.e., on "ord($char)%16".  In particular, characters "0" and "1"
               generate nybbles 0 and 1, as do bytes "\000" and "\001".  For characters "a".."f"
               and "A".."F", the result is compatible with the usual hexadecimal digits, so that
               "a" and "A" both generate the nybble "0xA==10".  Use only these specific hex
               characters with this format.

               Starting from the beginning of the template to "pack", each pair of characters is
               converted to 1 character of output.  With format "h", the first character of the
               pair determines the least-significant nybble of the output character; with format
               "H", it determines the most-significant nybble.

               If the length of the input string is not even, it behaves as if padded by a null
               character at the end.  Similarly, "extra" nybbles are ignored during unpacking.

               If the input string is longer than needed, extra characters are ignored.

               A "*" for the repeat count uses all characters of the input field.  For "unpack",
               nybbles are converted to a string of hexadecimal digits.

           ·   The "p" format packs a pointer to a null-terminated string.  You are responsible
               for ensuring that the string is not a temporary value, as that could potentially
               get deallocated before you got around to using the packed result.  The "P" format
               packs a pointer to a structure of the size indicated by the length.  A null
               pointer is created if the corresponding value for "p" or "P" is "undef"; similarly
               with "unpack", where a null pointer unpacks into "undef".

               If your system has a strange pointer size--meaning a pointer is neither as big as
               an int nor as big as a long--it may not be possible to pack or unpack pointers in
               big- or little-endian byte order.  Attempting to do so raises an exception.

           ·   The "/" template character allows packing and unpacking of a sequence of items
               where the packed structure contains a packed item count followed by the packed
               items themselves.  This is useful when the structure you're unpacking has encoded
               the sizes or repeat counts for some of its fields within the structure itself as
               separate fields.

               For "pack", you write length-item"/"sequence-item, and the length-item describes
               how the length value is packed.  Formats likely to be of most use are integer-
               packing ones like "n" for Java strings, "w" for ASN.1 or SNMP, and "N" for Sun
               XDR.

               For "pack", sequence-item may have a repeat count, in which case the minimum of
               that and the number of available items is used as the argument for length-item.
               If it has no repeat count or uses a '*', the number of available items is used.

               For "unpack", an internal stack of integer arguments unpacked so far is used.  You
               write "/"sequence-item and the repeat count is obtained by popping off the last
               element from the stack.  The sequence-item must not have a repeat count.

               If sequence-item refers to a string type ("A", "a", or "Z"), the length-item is
               the string length, not the number of strings.  With an explicit repeat count for
               pack, the packed string is adjusted to that length.  For example:

                This code:                             gives this result:

                unpack("W/a", "\004Gurusamy")          ("Guru")
                unpack("a3/A A*", "007 Bond  J ")      (" Bond", "J")
                unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")

                pack("n/a* w/a","hello,","world")     "\000\006hello,\005world"
                pack("a/W2", ord("a") .. ord("z"))    "2ab"

               The length-item is not returned explicitly from "unpack".

               Supplying a count to the length-item format letter is only useful with "A", "a",
               or "Z".  Packing with a length-item of "a" or "Z" may introduce "\000" characters,
               which Perl does not regard as legal in numeric strings.

           ·   The integer types "s", "S", "l", and "L" may be followed by a "!" modifier to
               specify native shorts or longs.  As shown in the example above, a bare "l" means
               exactly 32 bits, although the native "long" as seen by the local C compiler may be
               larger.  This is mainly an issue on 64-bit platforms.  You can see whether using
               "!" makes any difference this way:

                   printf "format s is %d, s! is %d\n",
                       length pack("s"), length pack("s!");

                   printf "format l is %d, l! is %d\n",
                       length pack("l"), length pack("l!");

               "i!" and "I!" are also allowed, but only for completeness' sake: they are
               identical to "i" and "I".

               The actual sizes (in bytes) of native shorts, ints, longs, and long longs on the
               platform where Perl was built are also available from the command line:

                   $ perl -V:{short,int,long{,long}}size
                   shortsize='2';
                   intsize='4';
                   longsize='4';
                   longlongsize='8';

               or programmatically via the "Config" module:

                      use Config;
                      print $Config{shortsize},    "\n";
                      print $Config{intsize},      "\n";
                      print $Config{longsize},     "\n";
                      print $Config{longlongsize}, "\n";

               $Config{longlongsize} is undefined on systems without long long support.

           ·   The integer formats "s", "S", "i", "I", "l", "L", "j", and "J" are inherently non-
               portable between processors and operating systems because they obey native
               byteorder and endianness.  For example, a 4-byte integer 0x12345678 (305419896
               decimal) would be ordered natively (arranged in and handled by the CPU registers)
               into bytes as

                   0x12 0x34 0x56 0x78  # big-endian
                   0x78 0x56 0x34 0x12  # little-endian

               Basically, Intel and VAX CPUs are little-endian, while everybody else, including
               Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are big-endian.  Alpha and
               MIPS can be either: Digital/Compaq uses (well, used) them in little-endian mode,
               but SGI/Cray uses them in big-endian mode.

               The names big-endian and little-endian are comic references to the egg-eating
               habits of the little-endian Lilliputians and the big-endian Blefuscudians from the
               classic Jonathan Swift satire, Gulliver's Travels.  This entered computer lingo
               via the paper "On Holy Wars and a Plea for Peace" by Danny Cohen, USC/ISI IEN 137,
               April 1, 1980.

               Some systems may have even weirder byte orders such as

                  0x56 0x78 0x12 0x34
                  0x34 0x12 0x78 0x56

               These are called mid-endian, middle-endian, mixed-endian, or just weird.

               You can determine your system endianness with this incantation:

                  printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);

               The byteorder on the platform where Perl was built is also available via Config:

                   use Config;
                   print "$Config{byteorder}\n";

               or from the command line:

                   $ perl -V:byteorder

               Byteorders "1234" and "12345678" are little-endian; "4321" and "87654321" are big-
               endian.  Systems with multiarchitecture binaries will have "ffff", signifying that
               static information doesn't work, one must use runtime probing.

               For portably packed integers, either use the formats "n", "N", "v", and "V" or
               else use the ">" and "<" modifiers described immediately below.  See also
               perlport.

           ·   Also floating point numbers have endianness.  Usually (but not always) this agrees
               with the integer endianness.  Even though most platforms these days use the IEEE
               754 binary format, there are differences, especially if the long doubles are
               involved.  You can see the "Config" variables "doublekind" and "longdblkind" (also
               "doublesize", "longdblsize"): the "kind" values are enums, unlike "byteorder".

               Portability-wise the best option is probably to keep to the IEEE 754 64-bit
               doubles, and of agreed-upon endianness.  Another possibility is the "%a") format
               of "printf".

           ·   Starting with Perl 5.10.0, integer and floating-point formats, along with the "p"
               and "P" formats and "()" groups, may all be followed by the ">" or "<" endianness
               modifiers to respectively enforce big- or little-endian byte-order.  These
               modifiers are especially useful given how "n", "N", "v", and "V" don't cover
               signed integers, 64-bit integers, or floating-point values.

               Here are some concerns to keep in mind when using an endianness modifier:

               ·   Exchanging signed integers between different platforms works only when all
                   platforms store them in the same format.  Most platforms store signed integers
                   in two's-complement notation, so usually this is not an issue.

               ·   The ">" or "<" modifiers can only be used on floating-point formats on big- or
                   little-endian machines.  Otherwise, attempting to use them raises an
                   exception.

               ·   Forcing big- or little-endian byte-order on floating-point values for data
                   exchange can work only if all platforms use the same binary representation
                   such as IEEE floating-point.  Even if all platforms are using IEEE, there may
                   still be subtle differences.  Being able to use ">" or "<" on floating-point
                   values can be useful, but also dangerous if you don't know exactly what you're
                   doing.  It is not a general way to portably store floating-point values.

               ·   When using ">" or "<" on a "()" group, this affects all types inside the group
                   that accept byte-order modifiers, including all subgroups.  It is silently
                   ignored for all other types.  You are not allowed to override the byte-order
                   within a group that already has a byte-order modifier suffix.

           ·   Real numbers (floats and doubles) are in native machine format only.  Due to the
               multiplicity of floating-point formats and the lack of a standard "network"
               representation for them, no facility for interchange has been made.  This means
               that packed floating-point data written on one machine may not be readable on
               another, even if both use IEEE floating-point arithmetic (because the endianness
               of the memory representation is not part of the IEEE spec).  See also perlport.

               If you know exactly what you're doing, you can use the ">" or "<" modifiers to
               force big- or little-endian byte-order on floating-point values.

               Because Perl uses doubles (or long doubles, if configured) internally for all
               numeric calculation, converting from double into float and thence to double again
               loses precision, so "unpack("f", pack("f", $foo)") will not in general equal $foo.

           ·   Pack and unpack can operate in two modes: character mode ("C0" mode) where the
               packed string is processed per character, and UTF-8 byte mode ("U0" mode) where
               the packed string is processed in its UTF-8-encoded Unicode form on a byte-by-byte
               basis.  Character mode is the default unless the format string starts with "U".
               You can always switch mode mid-format with an explicit "C0" or "U0" in the format.
               This mode remains in effect until the next mode change, or until the end of the
               "()" group it (directly) applies to.

               Using "C0" to get Unicode characters while using "U0" to get non-Unicode bytes is
               not necessarily obvious.   Probably only the first of these is what you want:

                   $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
                     perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
                   03B1.03C9
                   $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
                     perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
                   CE.B1.CF.89
                   $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
                     perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
                   CE.B1.CF.89
                   $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
                     perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
                   C3.8E.C2.B1.C3.8F.C2.89

               Those examples also illustrate that you should not try to use "pack"/"unpack" as a
               substitute for the Encode module.

           ·   You must yourself do any alignment or padding by inserting, for example, enough
               "x"es while packing.  There is no way for "pack" and "unpack" to know where
               characters are going to or coming from, so they handle their output and input as
               flat sequences of characters.

           ·   A "()" group is a sub-TEMPLATE enclosed in parentheses.  A group may take a repeat
               count either as postfix, or for "unpack", also via the "/" template character.
               Within each repetition of a group, positioning with "@" starts over at 0.
               Therefore, the result of

                   pack("@1A((@2A)@3A)", qw[X Y Z])

               is the string "\0X\0\0YZ".

           ·   "x" and "X" accept the "!" modifier to act as alignment commands: they jump
               forward or back to the closest position aligned at a multiple of "count"
               characters.  For example, to "pack" or "unpack" a C structure like

                   struct {
                       char   c;    /* one signed, 8-bit character */
                       double d;
                       char   cc[2];
                   }

               one may need to use the template "c x![d] d c[2]".  This assumes that doubles must
               be aligned to the size of double.

               For alignment commands, a "count" of 0 is equivalent to a "count" of 1; both are
               no-ops.

           ·   "n", "N", "v" and "V" accept the "!" modifier to represent signed 16-/32-bit
               integers in big-/little-endian order.  This is portable only when all platforms
               sharing packed data use the same binary representation for signed integers; for
               example, when all platforms use two's-complement representation.

           ·   Comments can be embedded in a TEMPLATE using "#" through the end of line.  White
               space can separate pack codes from each other, but modifiers and repeat counts
               must follow immediately.  Breaking complex templates into individual line-by-line
               components, suitably annotated, can do as much to improve legibility and
               maintainability of pack/unpack formats as "/x" can for complicated pattern
               matches.

           ·   If TEMPLATE requires more arguments than "pack" is given, "pack" assumes
               additional "" arguments.  If TEMPLATE requires fewer arguments than given, extra
               arguments are ignored.

           ·   Attempting to pack the special floating point values "Inf" and "NaN" (infinity,
               also in negative, and not-a-number) into packed integer values (like "L") is a
               fatal error.  The reason for this is that there simply isn't any sensible mapping
               for these special values into integers.

           Examples:

               $foo = pack("WWWW",65,66,67,68);
               # foo eq "ABCD"
               $foo = pack("W4",65,66,67,68);
               # same thing
               $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
               # same thing with Unicode circled letters.
               $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
               # same thing with Unicode circled letters.  You don't get the
               # UTF-8 bytes because the U at the start of the format caused
               # a switch to U0-mode, so the UTF-8 bytes get joined into
               # characters
               $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
               # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
               # This is the UTF-8 encoding of the string in the
               # previous example

               $foo = pack("ccxxcc",65,66,67,68);
               # foo eq "AB\0\0CD"

               # NOTE: The examples above featuring "W" and "c" are true
               # only on ASCII and ASCII-derived systems such as ISO Latin 1
               # and UTF-8.  On EBCDIC systems, the first example would be
               #      $foo = pack("WWWW",193,194,195,196);

               $foo = pack("s2",1,2);
               # "\001\000\002\000" on little-endian
               # "\000\001\000\002" on big-endian

               $foo = pack("a4","abcd","x","y","z");
               # "abcd"

               $foo = pack("aaaa","abcd","x","y","z");
               # "axyz"

               $foo = pack("a14","abcdefg");
               # "abcdefg\0\0\0\0\0\0\0"

               $foo = pack("i9pl", gmtime);
               # a real struct tm (on my system anyway)

               $utmp_template = "Z8 Z8 Z16 L";
               $utmp = pack($utmp_template, @utmp1);
               # a struct utmp (BSDish)

               @utmp2 = unpack($utmp_template, $utmp);
               # "@utmp1" eq "@utmp2"

               sub bintodec {
                   unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
               }

               $foo = pack('sx2l', 12, 34);
               # short 12, two zero bytes padding, long 34
               $bar = pack('s@4l', 12, 34);
               # short 12, zero fill to position 4, long 34
               # $foo eq $bar
               $baz = pack('s.l', 12, 4, 34);
               # short 12, zero fill to position 4, long 34

               $foo = pack('nN', 42, 4711);
               # pack big-endian 16- and 32-bit unsigned integers
               $foo = pack('S>L>', 42, 4711);
               # exactly the same
               $foo = pack('s<l<', -42, 4711);
               # pack little-endian 16- and 32-bit signed integers
               $foo = pack('(sl)<', -42, 4711);
               # exactly the same

           The same template may generally also be used in "unpack".

       package NAMESPACE
       package NAMESPACE VERSION
       package NAMESPACE BLOCK
       package NAMESPACE VERSION BLOCK
           Declares the BLOCK or the rest of the compilation unit as being in the given
           namespace.  The scope of the package declaration is either the supplied code BLOCK or,
           in the absence of a BLOCK, from the declaration itself through the end of current
           scope (the enclosing block, file, or "eval").  That is, the forms without a BLOCK are
           operative through the end of the current scope, just like the "my", "state", and "our"
           operators.  All unqualified dynamic identifiers in this scope will be in the given
           namespace, except where overridden by another "package" declaration or when they're
           one of the special identifiers that qualify into "main::", like "STDOUT", "ARGV",
           "ENV", and the punctuation variables.

           A package statement affects dynamic variables only, including those you've used
           "local" on, but not lexically-scoped variables, which are created with "my", "state",
           or "our".  Typically it would be the first declaration in a file included by "require"
           or "use".  You can switch into a package in more than one place, since this only
           determines which default symbol table the compiler uses for the rest of that block.
           You can refer to identifiers in other packages than the current one by prefixing the
           identifier with the package name and a double colon, as in $SomePack::var or
           "ThatPack::INPUT_HANDLE".  If package name is omitted, the "main" package as assumed.
           That is, $::sail is equivalent to $main::sail (as well as to "$main'sail", still seen
           in ancient code, mostly from Perl 4).

           If VERSION is provided, "package" sets the $VERSION variable in the given namespace to
           a version object with the VERSION provided.  VERSION must be a "strict" style version
           number as defined by the version module: a positive decimal number (integer or
           decimal-fraction) without exponentiation or else a dotted-decimal v-string with a
           leading 'v' character and at least three components.  You should set $VERSION only
           once per package.

           See "Packages" in perlmod for more information about packages, modules, and classes.
           See perlsub for other scoping issues.

       __PACKAGE__
           A special token that returns the name of the package in which it occurs.

       pipe READHANDLE,WRITEHANDLE
           Opens a pair of connected pipes like the corresponding system call.  Note that if you
           set up a loop of piped processes, deadlock can occur unless you are very careful.  In
           addition, note that Perl's pipes use IO buffering, so you may need to set $| to flush
           your WRITEHANDLE after each command, depending on the application.

           Returns true on success.

           See IPC::Open2, IPC::Open3, and "Bidirectional Communication with Another Process" in
           perlipc for examples of such things.

           On systems that support a close-on-exec flag on files, that flag is set on all newly
           opened file descriptors whose "fileno"s are higher than the current value of $^F (by
           default 2 for "STDERR").  See "$^F" in perlvar.

       pop ARRAY
       pop Pops and returns the last value of the array, shortening the array by one element.

           Returns the undefined value if the array is empty, although this may also happen at
           other times.  If ARRAY is omitted, pops the @ARGV array in the main program, but the
           @_ array in subroutines, just like "shift".

           Starting with Perl 5.14, an experimental feature allowed "pop" to take a scalar
           expression. This experiment has been deemed unsuccessful, and was removed as of Perl
           5.24.

       pos SCALAR
       pos Returns the offset of where the last "m//g" search left off for the variable in
           question ($_ is used when the variable is not specified).  This offset is in
           characters unless the (no-longer-recommended) "use bytes" pragma is in effect, in
           which case the offset is in bytes.  Note that 0 is a valid match offset.  "undef"
           indicates that the search position is reset (usually due to match failure, but can
           also be because no match has yet been run on the scalar).

           "pos" directly accesses the location used by the regexp engine to store the offset, so
           assigning to "pos" will change that offset, and so will also influence the "\G" zero-
           width assertion in regular expressions.  Both of these effects take place for the next
           match, so you can't affect the position with "pos" during the current match, such as
           in "(?{pos() = 5})" or "s//pos() = 5/e".

           Setting "pos" also resets the matched with zero-length flag, described under "Repeated
           Patterns Matching a Zero-length Substring" in perlre.

           Because a failed "m//gc" match doesn't reset the offset, the return from "pos" won't
           change either in this case.  See perlre and perlop.

       print FILEHANDLE LIST
       print FILEHANDLE
       print LIST
       print
           Prints a string or a list of strings.  Returns true if successful.  FILEHANDLE may be
           a scalar variable containing the name of or a reference to the filehandle, thus
           introducing one level of indirection.  (NOTE: If FILEHANDLE is a variable and the next
           token is a term, it may be misinterpreted as an operator unless you interpose a "+" or
           put parentheses around the arguments.)  If FILEHANDLE is omitted, prints to the last
           selected (see "select") output handle.  If LIST is omitted, prints $_ to the currently
           selected output handle.  To use FILEHANDLE alone to print the content of $_ to it, you
           must use a bareword filehandle like "FH", not an indirect one like $fh.  To set the
           default output handle to something other than STDOUT, use the select operation.

           The current value of $, (if any) is printed between each LIST item.  The current value
           of "$\" (if any) is printed after the entire LIST has been printed.  Because print
           takes a LIST, anything in the LIST is evaluated in list context, including any
           subroutines whose return lists you pass to "print".  Be careful not to follow the
           print keyword with a left parenthesis unless you want the corresponding right
           parenthesis to terminate the arguments to the print; put parentheses around all
           arguments (or interpose a "+", but that doesn't look as good).

           If you're storing handles in an array or hash, or in general whenever you're using any
           expression more complex than a bareword handle or a plain, unsubscripted scalar
           variable to retrieve it, you will have to use a block returning the filehandle value
           instead, in which case the LIST may not be omitted:

               print { $files[$i] } "stuff\n";
               print { $OK ? *STDOUT : *STDERR } "stuff\n";

           Printing to a closed pipe or socket will generate a SIGPIPE signal.  See perlipc for
           more on signal handling.

       printf FILEHANDLE FORMAT, LIST
       printf FILEHANDLE
       printf FORMAT, LIST
       printf
           Equivalent to "print FILEHANDLE sprintf(FORMAT, LIST)", except that "$\" (the output
           record separator) is not appended.  The FORMAT and the LIST are actually parsed as a
           single list.  The first argument of the list will be interpreted as the "printf"
           format.  This means that "printf(@_)" will use $_[0] as the format.  See sprintf for
           an explanation of the format argument.  If "use locale" (including "use locale
           ':not_characters'") is in effect and "POSIX::setlocale" has been called, the character
           used for the decimal separator in formatted floating-point numbers is affected by the
           "LC_NUMERIC" locale setting.  See perllocale and POSIX.

           For historical reasons, if you omit the list, $_ is used as the format; to use
           FILEHANDLE without a list, you must use a bareword filehandle like "FH", not an
           indirect one like $fh.  However, this will rarely do what you want; if $_ contains
           formatting codes, they will be replaced with the empty string and a warning will be
           emitted if warnings are enabled.  Just use "print" if you want to print the contents
           of $_.

           Don't fall into the trap of using a "printf" when a simple "print" would do.  The
           "print" is more efficient and less error prone.

       prototype FUNCTION
       prototype
           Returns the prototype of a function as a string (or "undef" if the function has no
           prototype).  FUNCTION is a reference to, or the name of, the function whose prototype
           you want to retrieve.  If FUNCTION is omitted, $_ is used.

           If FUNCTION is a string starting with "CORE::", the rest is taken as a name for a Perl
           builtin.  If the builtin's arguments cannot be adequately expressed by a prototype
           (such as "system"), "prototype" returns "undef", because the builtin does not really
           behave like a Perl function.  Otherwise, the string describing the equivalent
           prototype is returned.

       push ARRAY,LIST
           Treats ARRAY as a stack by appending the values of LIST to the end of ARRAY.  The
           length of ARRAY increases by the length of LIST.  Has the same effect as

               for my $value (LIST) {
                   $ARRAY[++$#ARRAY] = $value;
               }

           but is more efficient.  Returns the number of elements in the array following the
           completed "push".

           Starting with Perl 5.14, an experimental feature allowed "push" to take a scalar
           expression. This experiment has been deemed unsuccessful, and was removed as of Perl
           5.24.

       q/STRING/
       qq/STRING/
       qw/STRING/
       qx/STRING/
           Generalized quotes.  See "Quote-Like Operators" in perlop.

       qr/STRING/
           Regexp-like quote.  See "Regexp Quote-Like Operators" in perlop.

       quotemeta EXPR
       quotemeta
           Returns the value of EXPR with all the ASCII non-"word" characters backslashed.  (That
           is, all ASCII characters not matching "/[A-Za-z_0-9]/" will be preceded by a backslash
           in the returned string, regardless of any locale settings.)  This is the internal
           function implementing the "\Q" escape in double-quoted strings.  (See below for the
           behavior on non-ASCII code points.)

           If EXPR is omitted, uses $_.

           quotemeta (and "\Q" ... "\E") are useful when interpolating strings into regular
           expressions, because by default an interpolated variable will be considered a mini-
           regular expression.  For example:

               my $sentence = 'The quick brown fox jumped over the lazy dog';
               my $substring = 'quick.*?fox';
               $sentence =~ s{$substring}{big bad wolf};

           Will cause $sentence to become 'The big bad wolf jumped over...'.

           On the other hand:

               my $sentence = 'The quick brown fox jumped over the lazy dog';
               my $substring = 'quick.*?fox';
               $sentence =~ s{\Q$substring\E}{big bad wolf};

           Or:

               my $sentence = 'The quick brown fox jumped over the lazy dog';
               my $substring = 'quick.*?fox';
               my $quoted_substring = quotemeta($substring);
               $sentence =~ s{$quoted_substring}{big bad wolf};

           Will both leave the sentence as is.  Normally, when accepting literal string input
           from the user, "quotemeta" or "\Q" must be used.

           In Perl v5.14, all non-ASCII characters are quoted in non-UTF-8-encoded strings, but
           not quoted in UTF-8 strings.

           Starting in Perl v5.16, Perl adopted a Unicode-defined strategy for quoting non-ASCII
           characters; the quoting of ASCII characters is unchanged.

           Also unchanged is the quoting of non-UTF-8 strings when outside the scope of a "use
           feature 'unicode_strings'", which is to quote all characters in the upper Latin1
           range.  This provides complete backwards compatibility for old programs which do not
           use Unicode.  (Note that "unicode_strings" is automatically enabled within the scope
           of a "use v5.12" or greater.)

           Within the scope of "use locale", all non-ASCII Latin1 code points are quoted whether
           the string is encoded as UTF-8 or not.  As mentioned above, locale does not affect the
           quoting of ASCII-range characters.  This protects against those locales where
           characters such as "|" are considered to be word characters.

           Otherwise, Perl quotes non-ASCII characters using an adaptation from Unicode (see
           <http://www.unicode.org/reports/tr31/>).  The only code points that are quoted are
           those that have any of the Unicode properties:  Pattern_Syntax, Pattern_White_Space,
           White_Space, Default_Ignorable_Code_Point, or General_Category=Control.

           Of these properties, the two important ones are Pattern_Syntax and
           Pattern_White_Space.  They have been set up by Unicode for exactly this purpose of
           deciding which characters in a regular expression pattern should be quoted.  No
           character that can be in an identifier has these properties.

           Perl promises, that if we ever add regular expression pattern metacharacters to the
           dozen already defined ("\ | ( ) [ { ^ $ * + ? ."), that we will only use ones that
           have the Pattern_Syntax property.  Perl also promises, that if we ever add characters
           that are considered to be white space in regular expressions (currently mostly
           affected by "/x"), they will all have the Pattern_White_Space property.

           Unicode promises that the set of code points that have these two properties will never
           change, so something that is not quoted in v5.16 will never need to be quoted in any
           future Perl release.  (Not all the code points that match Pattern_Syntax have actually
           had characters assigned to them; so there is room to grow, but they are quoted whether
           assigned or not.  Perl, of course, would never use an unassigned code point as an
           actual metacharacter.)

           Quoting characters that have the other 3 properties is done to enhance the readability
           of the regular expression and not because they actually need to be quoted for regular
           expression purposes (characters with the White_Space property are likely to be
           indistinguishable on the page or screen from those with the Pattern_White_Space
           property; and the other two properties contain non-printing characters).

       rand EXPR
       rand
           Returns a random fractional number greater than or equal to 0 and less than the value
           of EXPR.  (EXPR should be positive.)  If EXPR is omitted, the value 1 is used.
           Currently EXPR with the value 0 is also special-cased as 1 (this was undocumented
           before Perl 5.8.0 and is subject to change in future versions of Perl).  Automatically
           calls "srand" unless "srand" has already been called.  See also "srand".

           Apply "int" to the value returned by "rand" if you want random integers instead of
           random fractional numbers.  For example,

               int(rand(10))

           returns a random integer between 0 and 9, inclusive.

           (Note: If your rand function consistently returns numbers that are too large or too
           small, then your version of Perl was probably compiled with the wrong number of
           RANDBITS.)

           "rand" is not cryptographically secure.  You should not rely on it in security-
           sensitive situations.  As of this writing, a number of third-party CPAN modules offer
           random number generators intended by their authors to be cryptographically secure,
           including: Data::Entropy, Crypt::Random, Math::Random::Secure, and Math::TrulyRandom.

       read FILEHANDLE,SCALAR,LENGTH,OFFSET
       read FILEHANDLE,SCALAR,LENGTH
           Attempts to read LENGTH characters of data into variable SCALAR from the specified
           FILEHANDLE.  Returns the number of characters actually read, 0 at end of file, or
           undef if there was an error (in the latter case $! is also set).  SCALAR will be grown
           or shrunk so that the last character actually read is the last character of the scalar
           after the read.

           An OFFSET may be specified to place the read data at some place in the string other
           than the beginning.  A negative OFFSET specifies placement at that many characters
           counting backwards from the end of the string.  A positive OFFSET greater than the
           length of SCALAR results in the string being padded to the required size with "\0"
           bytes before the result of the read is appended.

           The call is implemented in terms of either Perl's or your system's native fread(3)
           library function.  To get a true read(2) system call, see sysread.

           Note the characters: depending on the status of the filehandle, either (8-bit) bytes
           or characters are read.  By default, all filehandles operate on bytes, but for example
           if the filehandle has been opened with the ":utf8" I/O layer (see "open", and the open
           pragma), the I/O will operate on UTF8-encoded Unicode characters, not bytes.
           Similarly for the ":encoding" layer: in that case pretty much any characters can be
           read.

       readdir DIRHANDLE
           Returns the next directory entry for a directory opened by "opendir".  If used in list
           context, returns all the rest of the entries in the directory.  If there are no more
           entries, returns the undefined value in scalar context and the empty list in list
           context.

           If you're planning to filetest the return values out of a "readdir", you'd better
           prepend the directory in question.  Otherwise, because we didn't "chdir" there, it
           would have been testing the wrong file.

               opendir(my $dh, $some_dir) || die "Can't opendir $some_dir: $!";
               my @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
               closedir $dh;

           As of Perl 5.12 you can use a bare "readdir" in a "while" loop, which will set $_ on
           every iteration.

               opendir(my $dh, $some_dir) || die "Can't open $some_dir: $!";
               while (readdir $dh) {
                   print "$some_dir/$_\n";
               }
               closedir $dh;

           To avoid confusing would-be users of your code who are running earlier versions of
           Perl with mysterious failures, put this sort of thing at the top of your file to
           signal that your code will work only on Perls of a recent vintage:

               use 5.012; # so readdir assigns to $_ in a lone while test

       readline EXPR
       readline
           Reads from the filehandle whose typeglob is contained in EXPR (or from *ARGV if EXPR
           is not provided).  In scalar context, each call reads and returns the next line until
           end-of-file is reached, whereupon the subsequent call returns "undef".  In list
           context, reads until end-of-file is reached and returns a list of lines.  Note that
           the notion of "line" used here is whatever you may have defined with $/ (or
           $INPUT_RECORD_SEPARATOR in English).  See "$/" in perlvar.

           When $/ is set to "undef", when "readline" is in scalar context (i.e., file slurp
           mode), and when an empty file is read, it returns '' the first time, followed by
           "undef" subsequently.

           This is the internal function implementing the "<EXPR>" operator, but you can use it
           directly.  The "<EXPR>" operator is discussed in more detail in "I/O Operators" in
           perlop.

               my $line = <STDIN>;
               my $line = readline(STDIN);    # same thing

           If "readline" encounters an operating system error, $! will be set with the
           corresponding error message.  It can be helpful to check $! when you are reading from
           filehandles you don't trust, such as a tty or a socket.  The following example uses
           the operator form of "readline" and dies if the result is not defined.

               while ( ! eof($fh) ) {
                   defined( $_ = readline $fh ) or die "readline failed: $!";
                   ...
               }

           Note that you have can't handle "readline" errors that way with the "ARGV" filehandle.
           In that case, you have to open each element of @ARGV yourself since "eof" handles
           "ARGV" differently.

               foreach my $arg (@ARGV) {
                   open(my $fh, $arg) or warn "Can't open $arg: $!";

                   while ( ! eof($fh) ) {
                       defined( $_ = readline $fh )
                           or die "readline failed for $arg: $!";
                       ...
                   }
               }

       readlink EXPR
       readlink
           Returns the value of a symbolic link, if symbolic links are implemented.  If not,
           raises an exception.  If there is a system error, returns the undefined value and sets
           $! (errno).  If EXPR is omitted, uses $_.

           Portability issues: "readlink" in perlport.

       readpipe EXPR
       readpipe
           EXPR is executed as a system command.  The collected standard output of the command is
           returned.  In scalar context, it comes back as a single (potentially multi-line)
           string.  In list context, returns a list of lines (however you've defined lines with
           $/ (or $INPUT_RECORD_SEPARATOR in English)).  This is the internal function
           implementing the "qx/EXPR/" operator, but you can use it directly.  The "qx/EXPR/"
           operator is discussed in more detail in "I/O Operators" in perlop.  If EXPR is
           omitted, uses $_.

       recv SOCKET,SCALAR,LENGTH,FLAGS
           Receives a message on a socket.  Attempts to receive LENGTH characters of data into
           variable SCALAR from the specified SOCKET filehandle.  SCALAR will be grown or shrunk
           to the length actually read.  Takes the same flags as the system call of the same
           name.  Returns the address of the sender if SOCKET's protocol supports this; returns
           an empty string otherwise.  If there's an error, returns the undefined value.  This
           call is actually implemented in terms of the recvfrom(2) system call.  See "UDP:
           Message Passing" in perlipc for examples.

           Note the characters: depending on the status of the socket, either (8-bit) bytes or
           characters are received.  By default all sockets operate on bytes, but for example if
           the socket has been changed using "binmode" to operate with the ":encoding(UTF-8)" I/O
           layer (see the open pragma), the I/O will operate on UTF8-encoded Unicode characters,
           not bytes.  Similarly for the ":encoding" layer: in that case pretty much any
           characters can be read.

       redo LABEL
       redo EXPR
       redo
           The "redo" command restarts the loop block without evaluating the conditional again.
           The "continue" block, if any, is not executed.  If the LABEL is omitted, the command
           refers to the innermost enclosing loop.  The "redo EXPR" form, available starting in
           Perl 5.18.0, allows a label name to be computed at run time, and is otherwise
           identical to "redo LABEL".  Programs that want to lie to themselves about what was
           just input normally use this command:

               # a simpleminded Pascal comment stripper
               # (warning: assumes no { or } in strings)
               LINE: while (<STDIN>) {
                   while (s|({.*}.*){.*}|$1 |) {}
                   s|{.*}| |;
                   if (s|{.*| |) {
                       my $front = $_;
                       while (<STDIN>) {
                           if (/}/) {  # end of comment?
                               s|^|$front\{|;
                               redo LINE;
                           }
                       }
                   }
                   print;
               }

           "redo" cannot be used to retry a block that returns a value such as "eval {}", "sub
           {}", or "do {}", and should not be used to exit a "grep" or "map" operation.

           Note that a block by itself is semantically identical to a loop that executes once.
           Thus "redo" inside such a block will effectively turn it into a looping construct.

           See also "continue" for an illustration of how "last", "next", and "redo" work.

           Unlike most named operators, this has the same precedence as assignment.  It is also
           exempt from the looks-like-a-function rule, so "redo ("foo")."bar"" will cause "bar"
           to be part of the argument to "redo".

       ref EXPR
       ref Returns a non-empty string if EXPR is a reference, the empty string otherwise.  If
           EXPR is not specified, $_ will be used.  The value returned depends on the type of
           thing the reference is a reference to.

           Builtin types include:

               SCALAR
               ARRAY
               HASH
               CODE
               REF
               GLOB
               LVALUE
               FORMAT
               IO
               VSTRING
               Regexp

           You can think of "ref" as a "typeof" operator.

               if (ref($r) eq "HASH") {
                   print "r is a reference to a hash.\n";
               }
               unless (ref($r)) {
                   print "r is not a reference at all.\n";
               }

           The return value "LVALUE" indicates a reference to an lvalue that is not a variable.
           You get this from taking the reference of function calls like "pos" or "substr".
           "VSTRING" is returned if the reference points to a version string.

           The result "Regexp" indicates that the argument is a regular expression resulting from
           "qr//".

           If the referenced object has been blessed into a package, then that package name is
           returned instead.  But don't use that, as it's now considered "bad practice".  For one
           reason, an object could be using a class called "Regexp" or "IO", or even "HASH".
           Also, "ref" doesn't take into account subclasses, like "isa" does.

           Instead, use "blessed" (in the Scalar::Util module) for boolean checks, "isa" for
           specific class checks and "reftype" (also from Scalar::Util) for type checks.  (See
           perlobj for details and a "blessed"/"isa" example.)

           See also perlref.

       rename OLDNAME,NEWNAME
           Changes the name of a file; an existing file NEWNAME will be clobbered.  Returns true
           for success, false otherwise.

           Behavior of this function varies wildly depending on your system implementation.  For
           example, it will usually not work across file system boundaries, even though the
           system mv command sometimes compensates for this.  Other restrictions include whether
           it works on directories, open files, or pre-existing files.  Check perlport and either
           the rename(2) manpage or equivalent system documentation for details.

           For a platform independent "move" function look at the File::Copy module.

           Portability issues: "rename" in perlport.

       require VERSION
       require EXPR
       require
           Demands a version of Perl specified by VERSION, or demands some semantics specified by
           EXPR or by $_ if EXPR is not supplied.

           VERSION may be either a numeric argument such as 5.006, which will be compared to $],
           or a literal of the form v5.6.1, which will be compared to $^V (or $PERL_VERSION in
           English).  An exception is raised if VERSION is greater than the version of the
           current Perl interpreter.  Compare with "use", which can do a similar check at compile
           time.

           Specifying VERSION as a literal of the form v5.6.1 should generally be avoided,
           because it leads to misleading error messages under earlier versions of Perl that do
           not support this syntax.  The equivalent numeric version should be used instead.

               require v5.6.1;     # run time version check
               require 5.6.1;      # ditto
               require 5.006_001;  # ditto; preferred for backwards
                                     compatibility

           Otherwise, "require" demands that a library file be included if it hasn't already been
           included.  The file is included via the do-FILE mechanism, which is essentially just a
           variety of "eval" with the caveat that lexical variables in the invoking script will
           be invisible to the included code.  If it were implemented in pure Perl, it would have
           semantics similar to the following:

               use Carp 'croak';
               use version;

               sub require {
                   my ($filename) = @_;
                   if ( my $version = eval { version->parse($filename) } ) {
                       if ( $version > $^V ) {
                          my $vn = $version->normal;
                          croak "Perl $vn required--this is only $^V, stopped";
                       }
                       return 1;
                   }

                   if (exists $INC{$filename}) {
                       return 1 if $INC{$filename};
                       croak "Compilation failed in require";
                   }

                   foreach $prefix (@INC) {
                       if (ref($prefix)) {
                           #... do other stuff - see text below ....
                       }
                       # (see text below about possible appending of .pmc
                       # suffix to $filename)
                       my $realfilename = "$prefix/$filename";
                       next if ! -e $realfilename || -d _ || -b _;
                       $INC{$filename} = $realfilename;
                       my $result = do($realfilename);
                                    # but run in caller's namespace

                       if (!defined $result) {
                           $INC{$filename} = undef;
                           croak $@ ? "$@Compilation failed in require"
                                    : "Can't locate $filename: $!\n";
                       }
                       if (!$result) {
                           delete $INC{$filename};
                           croak "$filename did not return true value";
                       }
                       $! = 0;
                       return $result;
                   }
                   croak "Can't locate $filename in \@INC ...";
               }

           Note that the file will not be included twice under the same specified name.

           The file must return true as the last statement to indicate successful execution of
           any initialization code, so it's customary to end such a file with "1;" unless you're
           sure it'll return true otherwise.  But it's better just to put the "1;", in case you
           add more statements.

           If EXPR is a bareword, "require" assumes a .pm extension and replaces "::" with "/" in
           the filename for you, to make it easy to load standard modules.  This form of loading
           of modules does not risk altering your namespace.

           In other words, if you try this:

                   require Foo::Bar;     # a splendid bareword

           The require function will actually look for the Foo/Bar.pm file in the directories
           specified in the @INC array.

           But if you try this:

                   my $class = 'Foo::Bar';
                   require $class;       # $class is not a bareword
               #or
                   require "Foo::Bar";   # not a bareword because of the ""

           The require function will look for the Foo::Bar file in the @INC  array and will
           complain about not finding Foo::Bar there.  In this case you can do:

                   eval "require $class";

           Now that you understand how "require" looks for files with a bareword argument, there
           is a little extra functionality going on behind the scenes.  Before "require" looks
           for a .pm extension, it will first look for a similar filename with a .pmc extension.
           If this file is found, it will be loaded in place of any file ending in a .pm
           extension.

           You can also insert hooks into the import facility by putting Perl code directly into
           the @INC array.  There are three forms of hooks: subroutine references, array
           references, and blessed objects.

           Subroutine references are the simplest case.  When the inclusion system walks through
           @INC and encounters a subroutine, this subroutine gets called with two parameters, the
           first a reference to itself, and the second the name of the file to be included (e.g.,
           Foo/Bar.pm).  The subroutine should return either nothing or else a list of up to four
           values in the following order:

           1.  A reference to a scalar, containing any initial source code to prepend to the file
               or generator output.

           2.  A filehandle, from which the file will be read.

           3.  A reference to a subroutine.  If there is no filehandle (previous item), then this
               subroutine is expected to generate one line of source code per call, writing the
               line into $_ and returning 1, then finally at end of file returning 0.  If there
               is a filehandle, then the subroutine will be called to act as a simple source
               filter, with the line as read in $_.  Again, return 1 for each valid line, and 0
               after all lines have been returned.

           4.  Optional state for the subroutine.  The state is passed in as $_[1].  A reference
               to the subroutine itself is passed in as $_[0].

           If an empty list, "undef", or nothing that matches the first 3 values above is
           returned, then "require" looks at the remaining elements of @INC.  Note that this
           filehandle must be a real filehandle (strictly a typeglob or reference to a typeglob,
           whether blessed or unblessed); tied filehandles will be ignored and processing will
           stop there.

           If the hook is an array reference, its first element must be a subroutine reference.
           This subroutine is called as above, but the first parameter is the array reference.
           This lets you indirectly pass arguments to the subroutine.

           In other words, you can write:

               push @INC, \&my_sub;
               sub my_sub {
                   my ($coderef, $filename) = @_;  # $coderef is \&my_sub
                   ...
               }

           or:

               push @INC, [ \&my_sub, $x, $y, ... ];
               sub my_sub {
                   my ($arrayref, $filename) = @_;
                   # Retrieve $x, $y, ...
                   my (undef, @parameters) = @$arrayref;
                   ...
               }

           If the hook is an object, it must provide an "INC" method that will be called as
           above, the first parameter being the object itself.  (Note that you must fully qualify
           the sub's name, as unqualified "INC" is always forced into package "main".)  Here is a
           typical code layout:

               # In Foo.pm
               package Foo;
               sub new { ... }
               sub Foo::INC {
                   my ($self, $filename) = @_;
                   ...
               }

               # In the main program
               push @INC, Foo->new(...);

           These hooks are also permitted to set the %INC entry corresponding to the files they
           have loaded.  See "%INC" in perlvar.

           For a yet-more-powerful import facility, see "use" and perlmod.

       reset EXPR
       reset
           Generally used in a "continue" block at the end of a loop to clear variables and reset
           "m?pattern?" searches so that they work again.  The expression is interpreted as a
           list of single characters (hyphens allowed for ranges).  All variables and arrays
           beginning with one of those letters are reset to their pristine state.  If the
           expression is omitted, one-match searches ("m?pattern?") are reset to match again.
           Only resets variables or searches in the current package.  Always returns 1.
           Examples:

               reset 'X';      # reset all X variables
               reset 'a-z';    # reset lower case variables
               reset;          # just reset m?one-time? searches

           Resetting "A-Z" is not recommended because you'll wipe out your @ARGV and @INC arrays
           and your %ENV hash.  Resets only package variables; lexical variables are unaffected,
           but they clean themselves up on scope exit anyway, so you'll probably want to use them
           instead.  See "my".

       return EXPR
       return
           Returns from a subroutine, "eval", "do FILE", "sort" block or regex eval block (but
           not a "grep" or "map" block) with the value given in EXPR.  Evaluation of EXPR may be
           in list, scalar, or void context, depending on how the return value will be used, and
           the context may vary from one execution to the next (see "wantarray").  If no EXPR is
           given, returns an empty list in list context, the undefined value in scalar context,
           and (of course) nothing at all in void context.

           (In the absence of an explicit "return", a subroutine, "eval", or "do FILE"
           automatically returns the value of the last expression evaluated.)

           Unlike most named operators, this is also exempt from the looks-like-a-function rule,
           so "return ("foo")."bar"" will cause "bar" to be part of the argument to "return".

       reverse LIST
           In list context, returns a list value consisting of the elements of LIST in the
           opposite order.  In scalar context, concatenates the elements of LIST and returns a
           string value with all characters in the opposite order.

               print join(", ", reverse "world", "Hello"); # Hello, world

               print scalar reverse "dlrow ,", "olleH";    # Hello, world

           Used without arguments in scalar context, "reverse" reverses $_.

               $_ = "dlrow ,olleH";
               print reverse;                         # No output, list context
               print scalar reverse;                  # Hello, world

           Note that reversing an array to itself (as in "@a = reverse @a") will preserve non-
           existent elements whenever possible; i.e., for non-magical arrays or for tied arrays
           with "EXISTS" and "DELETE" methods.

           This operator is also handy for inverting a hash, although there are some caveats.  If
           a value is duplicated in the original hash, only one of those can be represented as a
           key in the inverted hash.  Also, this has to unwind one hash and build a whole new
           one, which may take some time on a large hash, such as from a DBM file.

               my %by_name = reverse %by_address;  # Invert the hash

       rewinddir DIRHANDLE
           Sets the current position to the beginning of the directory for the "readdir" routine
           on DIRHANDLE.

           Portability issues: "rewinddir" in perlport.

       rindex STR,SUBSTR,POSITION
       rindex STR,SUBSTR
           Works just like "index" except that it returns the position of the last occurrence of
           SUBSTR in STR.  If POSITION is specified, returns the last occurrence beginning at or
           before that position.

       rmdir FILENAME
       rmdir
           Deletes the directory specified by FILENAME if that directory is empty.  If it
           succeeds it returns true; otherwise it returns false and sets $! (errno).  If FILENAME
           is omitted, uses $_.

           To remove a directory tree recursively ("rm -rf" on Unix) look at the "rmtree"
           function of the File::Path module.

       s///
           The substitution operator.  See "Regexp Quote-Like Operators" in perlop.

       say FILEHANDLE LIST
       say FILEHANDLE
       say LIST
       say Just like "print", but implicitly appends a newline.  "say LIST" is simply an
           abbreviation for "{ local $\ = "\n"; print LIST }".  To use FILEHANDLE without a LIST
           to print the contents of $_ to it, you must use a bareword filehandle like "FH", not
           an indirect one like $fh.

           "say" is available only if the "say" feature is enabled or if it is prefixed with
           "CORE::".  The "say" feature is enabled automatically with a "use v5.10" (or higher)
           declaration in the current scope.

       scalar EXPR
           Forces EXPR to be interpreted in scalar context and returns the value of EXPR.

               my @counts = ( scalar @a, scalar @b, scalar @c );

           There is no equivalent operator to force an expression to be interpolated in list
           context because in practice, this is never needed.  If you really wanted to do so,
           however, you could use the construction "@{[ (some expression) ]}", but usually a
           simple "(some expression)" suffices.

           Because "scalar" is a unary operator, if you accidentally use a parenthesized list for
           the EXPR, this behaves as a scalar comma expression, evaluating all but the last
           element in void context and returning the final element evaluated in scalar context.
           This is seldom what you want.

           The following single statement:

               print uc(scalar(foo(), $bar)), $baz;

           is the moral equivalent of these two:

               foo();
               print(uc($bar), $baz);

           See perlop for more details on unary operators and the comma operator, and perldata
           for details on evaluating a hash in scalar contex.

       seek FILEHANDLE,POSITION,WHENCE
           Sets FILEHANDLE's position, just like the fseek(3) call of C "stdio".  FILEHANDLE may
           be an expression whose value gives the name of the filehandle.  The values for WHENCE
           are 0 to set the new position in bytes to POSITION; 1 to set it to the current
           position plus POSITION; and 2 to set it to EOF plus POSITION, typically negative.  For
           WHENCE you may use the constants "SEEK_SET", "SEEK_CUR", and "SEEK_END" (start of the
           file, current position, end of the file) from the Fcntl module.  Returns 1 on success,
           false otherwise.

           Note the emphasis on bytes: even if the filehandle has been set to operate on
           characters (for example using the ":encoding(UTF-8)" I/O layer), the "seek", "tell",
           and "sysseek" family of functions use byte offsets, not character offsets, because
           seeking to a character offset would be very slow in a UTF-8 file.

           If you want to position the file for "sysread" or "syswrite", don't use "seek",
           because buffering makes its effect on the file's read-write position unpredictable and
           non-portable.  Use "sysseek" instead.

           Due to the rules and rigors of ANSI C, on some systems you have to do a seek whenever
           you switch between reading and writing.  Amongst other things, this may have the
           effect of calling stdio's clearerr(3).  A WHENCE of 1 ("SEEK_CUR") is useful for not
           moving the file position:

               seek($fh, 0, 1);

           This is also useful for applications emulating "tail -f".  Once you hit EOF on your
           read and then sleep for a while, you (probably) have to stick in a dummy "seek" to
           reset things.  The "seek" doesn't change the position, but it does clear the end-of-
           file condition on the handle, so that the next "readline FILE" makes Perl try again to
           read something.  (We hope.)

           If that doesn't work (some I/O implementations are particularly cantankerous), you
           might need something like this:

               for (;;) {
                   for ($curpos = tell($fh); $_ = readline($fh);
                        $curpos = tell($fh)) {
                       # search for some stuff and put it into files
                   }
                   sleep($for_a_while);
                   seek($fh, $curpos, 0);
               }

       seekdir DIRHANDLE,POS
           Sets the current position for the "readdir" routine on DIRHANDLE.  POS must be a value
           returned by "telldir".  "seekdir" also has the same caveats about possible directory
           compaction as the corresponding system library routine.

       select FILEHANDLE
       select
           Returns the currently selected filehandle.  If FILEHANDLE is supplied, sets the new
           current default filehandle for output.  This has two effects: first, a "write" or a
           "print" without a filehandle default to this FILEHANDLE.  Second, references to
           variables related to output will refer to this output channel.

           For example, to set the top-of-form format for more than one output channel, you might
           do the following:

               select(REPORT1);
               $^ = 'report1_top';
               select(REPORT2);
               $^ = 'report2_top';

           FILEHANDLE may be an expression whose value gives the name of the actual filehandle.
           Thus:

               my $oldfh = select(STDERR); $| = 1; select($oldfh);

           Some programmers may prefer to think of filehandles as objects with methods,
           preferring to write the last example as:

               STDERR->autoflush(1);

           (Prior to Perl version 5.14, you have to "use IO::Handle;" explicitly first.)

           Portability issues: "select" in perlport.

       select RBITS,WBITS,EBITS,TIMEOUT
           This calls the select(2) syscall with the bit masks specified, which can be
           constructed using "fileno" and "vec", along these lines:

               my $rin = my $win = my $ein = '';
               vec($rin, fileno(STDIN),  1) = 1;
               vec($win, fileno(STDOUT), 1) = 1;
               $ein = $rin | $win;

           If you want to select on many filehandles, you may wish to write a subroutine like
           this:

               sub fhbits {
                   my @fhlist = @_;
                   my $bits = "";
                   for my $fh (@fhlist) {
                       vec($bits, fileno($fh), 1) = 1;
                   }
                   return $bits;
               }
               my $rin = fhbits(\*STDIN, $tty, $mysock);

           The usual idiom is:

            my ($nfound, $timeleft) =
              select(my $rout = $rin, my $wout = $win, my $eout = $ein,
                                                                     $timeout);

           or to block until something becomes ready just do this

            my $nfound =
              select(my $rout = $rin, my $wout = $win, my $eout = $ein, undef);

           Most systems do not bother to return anything useful in $timeleft, so calling "select"
           in scalar context just returns $nfound.

           Any of the bit masks can also be "undef".  The timeout, if specified, is in seconds,
           which may be fractional.  Note: not all implementations are capable of returning the
           $timeleft.  If not, they always return $timeleft equal to the supplied $timeout.

           You can effect a sleep of 250 milliseconds this way:

               select(undef, undef, undef, 0.25);

           Note that whether "select" gets restarted after signals (say, SIGALRM) is
           implementation-dependent.  See also perlport for notes on the portability of "select".

           On error, "select" behaves just like select(2): it returns "-1" and sets $!.

           On some Unixes, select(2) may report a socket file descriptor as "ready for reading"
           even when no data is available, and thus any subsequent "read" would block.  This can
           be avoided if you always use "O_NONBLOCK" on the socket.  See select(2) and fcntl(2)
           for further details.

           The standard "IO::Select" module provides a user-friendlier interface to "select",
           mostly because it does all the bit-mask work for you.

           WARNING: One should not attempt to mix buffered I/O (like "read" or "readline") with
           "select", except as permitted by POSIX, and even then only on POSIX systems.  You have
           to use "sysread" instead.

           Portability issues: "select" in perlport.

       semctl ID,SEMNUM,CMD,ARG
           Calls the System V IPC function semctl(2).  You'll probably have to say

               use IPC::SysV;

           first to get the correct constant definitions.  If CMD is IPC_STAT or GETALL, then ARG
           must be a variable that will hold the returned semid_ds structure or semaphore value
           array.  Returns like "ioctl": the undefined value for error, ""0 but true"" for zero,
           or the actual return value otherwise.  The ARG must consist of a vector of native
           short integers, which may be created with "pack("s!",(0)x$nsem)".  See also "SysV IPC"
           in perlipc and the documentation for "IPC::SysV" and "IPC::Semaphore".

           Portability issues: "semctl" in perlport.

       semget KEY,NSEMS,FLAGS
           Calls the System V IPC function semget(2).  Returns the semaphore id, or the undefined
           value on error.  See also "SysV IPC" in perlipc and the documentation for "IPC::SysV"
           and "IPC::Semaphore".

           Portability issues: "semget" in perlport.

       semop KEY,OPSTRING
           Calls the System V IPC function semop(2) for semaphore operations such as signalling
           and waiting.  OPSTRING must be a packed array of semop structures.  Each semop
           structure can be generated with "pack("s!3", $semnum, $semop, $semflag)".  The length
           of OPSTRING implies the number of semaphore operations.  Returns true if successful,
           false on error.  As an example, the following code waits on semaphore $semnum of
           semaphore id $semid:

               my $semop = pack("s!3", $semnum, -1, 0);
               die "Semaphore trouble: $!\n" unless semop($semid, $semop);

           To signal the semaphore, replace "-1" with 1.  See also "SysV IPC" in perlipc and the
           documentation for "IPC::SysV" and "IPC::Semaphore".

           Portability issues: "semop" in perlport.

       send SOCKET,MSG,FLAGS,TO
       send SOCKET,MSG,FLAGS
           Sends a message on a socket.  Attempts to send the scalar MSG to the SOCKET
           filehandle.  Takes the same flags as the system call of the same name.  On unconnected
           sockets, you must specify a destination to send to, in which case it does a sendto(2)
           syscall.  Returns the number of characters sent, or the undefined value on error.  The
           sendmsg(2) syscall is currently unimplemented.  See "UDP: Message Passing" in perlipc
           for examples.

           Note the characters: depending on the status of the socket, either (8-bit) bytes or
           characters are sent.  By default all sockets operate on bytes, but for example if the
           socket has been changed using "binmode" to operate with the ":encoding(UTF-8)" I/O
           layer (see "open", or the open pragma), the I/O will operate on UTF-8 encoded Unicode
           characters, not bytes.  Similarly for the ":encoding" layer: in that case pretty much
           any characters can be sent.

       setpgrp PID,PGRP
           Sets the current process group for the specified PID, 0 for the current process.
           Raises an exception when used on a machine that doesn't implement POSIX setpgid(2) or
           BSD setpgrp(2).  If the arguments are omitted, it defaults to "0,0".  Note that the
           BSD 4.2 version of "setpgrp" does not accept any arguments, so only "setpgrp(0,0)" is
           portable.  See also "POSIX::setsid()".

           Portability issues: "setpgrp" in perlport.

       setpriority WHICH,WHO,PRIORITY
           Sets the current priority for a process, a process group, or a user.  (See
           setpriority(2).)  Raises an exception when used on a machine that doesn't implement
           setpriority(2).

           Portability issues: "setpriority" in perlport.

       setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
           Sets the socket option requested.  Returns "undef" on error.  Use integer constants
           provided by the "Socket" module for LEVEL and OPNAME.  Values for LEVEL can also be
           obtained from getprotobyname.  OPTVAL might either be a packed string or an integer.
           An integer OPTVAL is shorthand for pack("i", OPTVAL).

           An example disabling Nagle's algorithm on a socket:

               use Socket qw(IPPROTO_TCP TCP_NODELAY);
               setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);

           Portability issues: "setsockopt" in perlport.

       shift ARRAY
       shift
           Shifts the first value of the array off and returns it, shortening the array by 1 and
           moving everything down.  If there are no elements in the array, returns the undefined
           value.  If ARRAY is omitted, shifts the @_ array within the lexical scope of
           subroutines and formats, and the @ARGV array outside a subroutine and also within the
           lexical scopes established by the "eval STRING", "BEGIN {}", "INIT {}", "CHECK {}",
           "UNITCHECK {}", and "END {}" constructs.

           Starting with Perl 5.14, an experimental feature allowed "shift" to take a scalar
           expression. This experiment has been deemed unsuccessful, and was removed as of Perl
           5.24.

           See also "unshift", "push", and "pop".  "shift" and "unshift" do the same thing to the
           left end of an array that "pop" and "push" do to the right end.

       shmctl ID,CMD,ARG
           Calls the System V IPC function shmctl.  You'll probably have to say

               use IPC::SysV;

           first to get the correct constant definitions.  If CMD is "IPC_STAT", then ARG must be
           a variable that will hold the returned "shmid_ds" structure.  Returns like ioctl:
           "undef" for error; "0 but true" for zero; and the actual return value otherwise.  See
           also "SysV IPC" in perlipc and the documentation for "IPC::SysV".

           Portability issues: "shmctl" in perlport.

       shmget KEY,SIZE,FLAGS
           Calls the System V IPC function shmget.  Returns the shared memory segment id, or
           "undef" on error.  See also "SysV IPC" in perlipc and the documentation for
           "IPC::SysV".

           Portability issues: "shmget" in perlport.

       shmread ID,VAR,POS,SIZE
       shmwrite ID,STRING,POS,SIZE
           Reads or writes the System V shared memory segment ID starting at position POS for
           size SIZE by attaching to it, copying in/out, and detaching from it.  When reading,
           VAR must be a variable that will hold the data read.  When writing, if STRING is too
           long, only SIZE bytes are used; if STRING is too short, nulls are written to fill out
           SIZE bytes.  Return true if successful, false on error.  "shmread" taints the
           variable.  See also "SysV IPC" in perlipc and the documentation for "IPC::SysV" and
           the "IPC::Shareable" module from CPAN.

           Portability issues: "shmread" in perlport and "shmwrite" in perlport.

       shutdown SOCKET,HOW
           Shuts down a socket connection in the manner indicated by HOW, which has the same
           interpretation as in the syscall of the same name.

               shutdown($socket, 0);    # I/we have stopped reading data
               shutdown($socket, 1);    # I/we have stopped writing data
               shutdown($socket, 2);    # I/we have stopped using this socket

           This is useful with sockets when you want to tell the other side you're done writing
           but not done reading, or vice versa.  It's also a more insistent form of close because
           it also disables the file descriptor in any forked copies in other processes.

           Returns 1 for success; on error, returns "undef" if the first argument is not a valid
           filehandle, or returns 0 and sets $! for any other failure.

       sin EXPR
       sin Returns the sine of EXPR (expressed in radians).  If EXPR is omitted, returns sine of
           $_.

           For the inverse sine operation, you may use the "Math::Trig::asin" function, or use
           this relation:

               sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }

       sleep EXPR
       sleep
           Causes the script to sleep for (integer) EXPR seconds, or forever if no argument is
           given.  Returns the integer number of seconds actually slept.

           May be interrupted if the process receives a signal such as "SIGALRM".

               eval {
                   local $SIG{ALRM} = sub { die "Alarm!\n" };
                   sleep;
               };
               die $@ unless $@ eq "Alarm!\n";

           You probably cannot mix "alarm" and "sleep" calls, because "sleep" is often
           implemented using "alarm".

           On some older systems, it may sleep up to a full second less than what you requested,
           depending on how it counts seconds.  Most modern systems always sleep the full amount.
           They may appear to sleep longer than that, however, because your process might not be
           scheduled right away in a busy multitasking system.

           For delays of finer granularity than one second, the Time::HiRes module (from CPAN,
           and starting from Perl 5.8 part of the standard distribution) provides "usleep".  You
           may also use Perl's four-argument version of "select" leaving the first three
           arguments undefined, or you might be able to use the "syscall" interface to access
           setitimer(2) if your system supports it.  See perlfaq8 for details.

           See also the POSIX module's "pause" function.

       socket SOCKET,DOMAIN,TYPE,PROTOCOL
           Opens a socket of the specified kind and attaches it to filehandle SOCKET.  DOMAIN,
           TYPE, and PROTOCOL are specified the same as for the syscall of the same name.  You
           should "use Socket" first to get the proper definitions imported.  See the examples in
           "Sockets: Client/Server Communication" in perlipc.

           On systems that support a close-on-exec flag on files, the flag will be set for the
           newly opened file descriptor, as determined by the value of $^F.  See "$^F" in
           perlvar.

       socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
           Creates an unnamed pair of sockets in the specified domain, of the specified type.
           DOMAIN, TYPE, and PROTOCOL are specified the same as for the syscall of the same name.
           If unimplemented, raises an exception.  Returns true if successful.

           On systems that support a close-on-exec flag on files, the flag will be set for the
           newly opened file descriptors, as determined by the value of $^F.  See "$^F" in
           perlvar.

           Some systems define "pipe" in terms of "socketpair", in which a call to "pipe($rdr,
           $wtr)" is essentially:

               use Socket;
               socketpair(my $rdr, my $wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
               shutdown($rdr, 1);        # no more writing for reader
               shutdown($wtr, 0);        # no more reading for writer

           See perlipc for an example of socketpair use.  Perl 5.8 and later will emulate
           socketpair using IP sockets to localhost if your system implements sockets but not
           socketpair.

           Portability issues: "socketpair" in perlport.

       sort SUBNAME LIST
       sort BLOCK LIST
       sort LIST
           In list context, this sorts the LIST and returns the sorted list value.  In scalar
           context, the behaviour of "sort" is undefined.

           If SUBNAME or BLOCK is omitted, "sort"s in standard string comparison order.  If
           SUBNAME is specified, it gives the name of a subroutine that returns an integer less
           than, equal to, or greater than 0, depending on how the elements of the list are to be
           ordered.  (The "<=>" and "cmp" operators are extremely useful in such routines.)
           SUBNAME may be a scalar variable name (unsubscripted), in which case the value
           provides the name of (or a reference to) the actual subroutine to use.  In place of a
           SUBNAME, you can provide a BLOCK as an anonymous, in-line sort subroutine.

           If the subroutine's prototype is "($$)", the elements to be compared are passed by
           reference in @_, as for a normal subroutine.  This is slower than unprototyped
           subroutines, where the elements to be compared are passed into the subroutine as the
           package global variables $a and $b (see example below).

           If the subroutine is an XSUB, the elements to be compared are pushed on to the stack,
           the way arguments are usually passed to XSUBs.  $a and $b are not set.

           The values to be compared are always passed by reference and should not be modified.

           You also cannot exit out of the sort block or subroutine using any of the loop control
           operators described in perlsyn or with "goto".

           When "use locale" (but not "use locale ':not_characters'") is in effect, "sort LIST"
           sorts LIST according to the current collation locale.  See perllocale.

           "sort" returns aliases into the original list, much as a for loop's index variable
           aliases the list elements.  That is, modifying an element of a list returned by "sort"
           (for example, in a "foreach", "map" or "grep") actually modifies the element in the
           original list.  This is usually something to be avoided when writing clear code.

           Perl 5.6 and earlier used a quicksort algorithm to implement sort.  That algorithm was
           not stable and could go quadratic.  (A stable sort preserves the input order of
           elements that compare equal.  Although quicksort's run time is O(NlogN) when averaged
           over all arrays of length N, the time can be O(N**2), quadratic behavior, for some
           inputs.)  In 5.7, the quicksort implementation was replaced with a stable mergesort
           algorithm whose worst-case behavior is O(NlogN).  But benchmarks indicated that for
           some inputs, on some platforms, the original quicksort was faster.  5.8 has a sort
           pragma for limited control of the sort.  Its rather blunt control of the underlying
           algorithm may not persist into future Perls, but the ability to characterize the input
           or output in implementation independent ways quite probably will.

           Examples:

               # sort lexically
               my @articles = sort @files;

               # same thing, but with explicit sort routine
               my @articles = sort {$a cmp $b} @files;

               # now case-insensitively
               my @articles = sort {fc($a) cmp fc($b)} @files;

               # same thing in reversed order
               my @articles = sort {$b cmp $a} @files;

               # sort numerically ascending
               my @articles = sort {$a <=> $b} @files;

               # sort numerically descending
               my @articles = sort {$b <=> $a} @files;

               # this sorts the %age hash by value instead of key
               # using an in-line function
               my @eldest = sort { $age{$b} <=> $age{$a} } keys %age;

               # sort using explicit subroutine name
               sub byage {
                   $age{$a} <=> $age{$b};  # presuming numeric
               }
               my @sortedclass = sort byage @class;

               sub backwards { $b cmp $a }
               my @harry  = qw(dog cat x Cain Abel);
               my @george = qw(gone chased yz Punished Axed);
               print sort @harry;
                   # prints AbelCaincatdogx
               print sort backwards @harry;
                   # prints xdogcatCainAbel
               print sort @george, 'to', @harry;
                   # prints AbelAxedCainPunishedcatchaseddoggonetoxyz

               # inefficiently sort by descending numeric compare using
               # the first integer after the first = sign, or the
               # whole record case-insensitively otherwise

               my @new = sort {
                   ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
                                       ||
                               fc($a)  cmp  fc($b)
               } @old;

               # same thing, but much more efficiently;
               # we'll build auxiliary indices instead
               # for speed
               my (@nums, @caps);
               for (@old) {
                   push @nums, ( /=(\d+)/ ? $1 : undef );
                   push @caps, fc($_);
               }

               my @new = @old[ sort {
                                      $nums[$b] <=> $nums[$a]
                                               ||
                                      $caps[$a] cmp $caps[$b]
                                    } 0..$#old
                             ];

               # same thing, but without any temps
               my @new = map { $_->[0] }
                      sort { $b->[1] <=> $a->[1]
                                      ||
                             $a->[2] cmp $b->[2]
                      } map { [$_, /=(\d+)/, fc($_)] } @old;

               # using a prototype allows you to use any comparison subroutine
               # as a sort subroutine (including other package's subroutines)
               package Other;
               sub backwards ($$) { $_[1] cmp $_[0]; }  # $a and $b are
                                                        # not set here
               package main;
               my @new = sort Other::backwards @old;

               # guarantee stability, regardless of algorithm
               use sort 'stable';
               my @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;

               # force use of mergesort (not portable outside Perl 5.8)
               use sort '_mergesort';  # note discouraging _
               my @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;

           Warning: syntactical care is required when sorting the list returned from a function.
           If you want to sort the list returned by the function call "find_records(@key)", you
           can use:

               my @contact = sort { $a cmp $b } find_records @key;
               my @contact = sort +find_records(@key);
               my @contact = sort &find_records(@key);
               my @contact = sort(find_records(@key));

           If instead you want to sort the array @key with the comparison routine
           "find_records()" then you can use:

               my @contact = sort { find_records() } @key;
               my @contact = sort find_records(@key);
               my @contact = sort(find_records @key);
               my @contact = sort(find_records (@key));

           $a and $b are set as package globals in the package the sort() is called from.  That
           means $main::a and $main::b (or $::a and $::b) in the "main" package, $FooPack::a and
           $FooPack::b in the "FooPack" package, etc.  If the sort block is in scope of a "my" or
           "state" declaration of $a and/or $b, you must spell out the full name of the variables
           in the sort block :

              package main;
              my $a = "C"; # DANGER, Will Robinson, DANGER !!!

              print sort { $a cmp $b }               qw(A C E G B D F H);
                                                     # WRONG
              sub badlexi { $a cmp $b }
              print sort badlexi                     qw(A C E G B D F H);
                                                     # WRONG
              # the above prints BACFEDGH or some other incorrect ordering

              print sort { $::a cmp $::b }           qw(A C E G B D F H);
                                                     # OK
              print sort { our $a cmp our $b }       qw(A C E G B D F H);
                                                     # also OK
              print sort { our ($a, $b); $a cmp $b } qw(A C E G B D F H);
                                                     # also OK
              sub lexi { our $a cmp our $b }
              print sort lexi                        qw(A C E G B D F H);
                                                     # also OK
              # the above print ABCDEFGH

           With proper care you may mix package and my (or state) $a and/or $b:

              my $a = {
                 tiny   => -2,
                 small  => -1,
                 normal => 0,
                 big    => 1,
                 huge   => 2
              };

              say sort { $a->{our $a} <=> $a->{our $b} }
                  qw{ huge normal tiny small big};

              # prints tinysmallnormalbighuge

           $a and $b are implicitely local to the sort() execution and regain their former values
           upon completing the sort.

           Sort subroutines written using $a and $b are bound to their calling package. It is
           possible, but of limited interest, to define them in a different package, since the
           subroutine must still refer to the calling package's $a and $b :

              package Foo;
              sub lexi { $Bar::a cmp $Bar::b }
              package Bar;
              ... sort Foo::lexi ...

           Use the prototyped versions (see above) for a more generic alternative.

           The comparison function is required to behave.  If it returns inconsistent results
           (sometimes saying $x[1] is less than $x[2] and sometimes saying the opposite, for
           example) the results are not well-defined.

           Because "<=>" returns "undef" when either operand is "NaN" (not-a-number), be careful
           when sorting with a comparison function like "$a <=> $b" any lists that might contain
           a "NaN".  The following example takes advantage that "NaN != NaN" to eliminate any
           "NaN"s from the input list.

               my @result = sort { $a <=> $b } grep { $_ == $_ } @input;

       splice ARRAY,OFFSET,LENGTH,LIST
       splice ARRAY,OFFSET,LENGTH
       splice ARRAY,OFFSET
       splice ARRAY
           Removes the elements designated by OFFSET and LENGTH from an array, and replaces them
           with the elements of LIST, if any.  In list context, returns the elements removed from
           the array.  In scalar context, returns the last element removed, or "undef" if no
           elements are removed.  The array grows or shrinks as necessary.  If OFFSET is negative
           then it starts that far from the end of the array.  If LENGTH is omitted, removes
           everything from OFFSET onward.  If LENGTH is negative, removes the elements from
           OFFSET onward except for -LENGTH elements at the end of the array.  If both OFFSET and
           LENGTH are omitted, removes everything.  If OFFSET is past the end of the array and a
           LENGTH was provided, Perl issues a warning, and splices at the end of the array.

           The following equivalences hold (assuming "$#a >= $i" )

               push(@a,$x,$y)      splice(@a,@a,0,$x,$y)
               pop(@a)             splice(@a,-1)
               shift(@a)           splice(@a,0,1)
               unshift(@a,$x,$y)   splice(@a,0,0,$x,$y)
               $a[$i] = $y         splice(@a,$i,1,$y)

           "splice" can be used, for example, to implement n-ary queue processing:

               sub nary_print {
                 my $n = shift;
                 while (my @next_n = splice @_, 0, $n) {
                   say join q{ -- }, @next_n;
                 }
               }

               nary_print(3, qw(a b c d e f g h));
               # prints:
               #   a -- b -- c
               #   d -- e -- f
               #   g -- h

           Starting with Perl 5.14, an experimental feature allowed "splice" to take a scalar
           expression. This experiment has been deemed unsuccessful, and was removed as of Perl
           5.24.

       split /PATTERN/,EXPR,LIMIT
       split /PATTERN/,EXPR
       split /PATTERN/
       split
           Splits the string EXPR into a list of strings and returns the list in list context, or
           the size of the list in scalar context.  (Prior to Perl 5.11, it also overwrote @_
           with the list in void and scalar context. If you target old perls, beware.)

           If only PATTERN is given, EXPR defaults to $_.

           Anything in EXPR that matches PATTERN is taken to be a separator that separates the
           EXPR into substrings (called "fields") that do not include the separator.  Note that a
           separator may be longer than one character or even have no characters at all (the
           empty string, which is a zero-width match).

           The PATTERN need not be constant; an expression may be used to specify a pattern that
           varies at runtime.

           If PATTERN matches the empty string, the EXPR is split at the match position (between
           characters).  As an example, the following:

               print join(':', split(/b/, 'abc')), "\n";

           uses the "b" in 'abc' as a separator to produce the output "a:c".  However, this:

               print join(':', split(//, 'abc')), "\n";

           uses empty string matches as separators to produce the output "a:b:c"; thus, the empty
           string may be used to split EXPR into a list of its component characters.

           As a special case for "split", the empty pattern given in match operator syntax ("//")
           specifically matches the empty string, which is contrary to its usual interpretation
           as the last successful match.

           If PATTERN is "/^/", then it is treated as if it used the multiline modifier ("/^/m"),
           since it isn't much use otherwise.

           "/m" and any of the other pattern modifiers valid for "qr" (summarized in
           "qr/STRING/msixpodualn" in perlop) may be specified explicitly.

           As another special case, "split" emulates the default behavior of the command line
           tool awk when the PATTERN is either omitted or a string composed of a single space
           character (such as ' ' or "\x20", but not e.g. "/ /").  In this case, any leading
           whitespace in EXPR is removed before splitting occurs, and the PATTERN is instead
           treated as if it were "/\s+/"; in particular, this means that any contiguous
           whitespace (not just a single space character) is used as a separator.  However, this
           special treatment can be avoided by specifying the pattern "/ /" instead of the string
           " ", thereby allowing only a single space character to be a separator.  In earlier
           Perls this special case was restricted to the use of a plain " " as the pattern
           argument to split; in Perl 5.18.0 and later this special case is triggered by any
           expression which evaluates to the simple string " ".

           If omitted, PATTERN defaults to a single space, " ", triggering the previously
           described awk emulation.

           If LIMIT is specified and positive, it represents the maximum number of fields into
           which the EXPR may be split; in other words, LIMIT is one greater than the maximum
           number of times EXPR may be split.  Thus, the LIMIT value 1 means that EXPR may be
           split a maximum of zero times, producing a maximum of one field (namely, the entire
           value of EXPR).  For instance:

               print join(':', split(//, 'abc', 1)), "\n";

           produces the output "abc", and this:

               print join(':', split(//, 'abc', 2)), "\n";

           produces the output "a:bc", and each of these:

               print join(':', split(//, 'abc', 3)), "\n";
               print join(':', split(//, 'abc', 4)), "\n";

           produces the output "a:b:c".

           If LIMIT is negative, it is treated as if it were instead arbitrarily large; as many
           fields as possible are produced.

           If LIMIT is omitted (or, equivalently, zero), then it is usually treated as if it were
           instead negative but with the exception that trailing empty fields are stripped (empty
           leading fields are always preserved); if all fields are empty, then all fields are
           considered to be trailing (and are thus stripped in this case).  Thus, the following:

               print join(':', split(/,/, 'a,b,c,,,')), "\n";

           produces the output "a:b:c", but the following:

               print join(':', split(/,/, 'a,b,c,,,', -1)), "\n";

           produces the output "a:b:c:::".

           In time-critical applications, it is worthwhile to avoid splitting into more fields
           than necessary.  Thus, when assigning to a list, if LIMIT is omitted (or zero), then
           LIMIT is treated as though it were one larger than the number of variables in the
           list; for the following, LIMIT is implicitly 3:

               my ($login, $passwd) = split(/:/);

           Note that splitting an EXPR that evaluates to the empty string always produces zero
           fields, regardless of the LIMIT specified.

           An empty leading field is produced when there is a positive-width match at the
           beginning of EXPR.  For instance:

               print join(':', split(/ /, ' abc')), "\n";

           produces the output ":abc".  However, a zero-width match at the beginning of EXPR
           never produces an empty field, so that:

               print join(':', split(//, ' abc'));

           produces the output " :a:b:c" (rather than ": :a:b:c").

           An empty trailing field, on the other hand, is produced when there is a match at the
           end of EXPR, regardless of the length of the match (of course, unless a non-zero LIMIT
           is given explicitly, such fields are removed, as in the last example).  Thus:

               print join(':', split(//, ' abc', -1)), "\n";

           produces the output " :a:b:c:".

           If the PATTERN contains capturing groups, then for each separator, an additional field
           is produced for each substring captured by a group (in the order in which the groups
           are specified, as per backreferences); if any group does not match, then it captures
           the "undef" value instead of a substring.  Also, note that any such additional field
           is produced whenever there is a separator (that is, whenever a split occurs), and such
           an additional field does not count towards the LIMIT.  Consider the following
           expressions evaluated in list context (each returned list is provided in the
           associated comment):

               split(/-|,/, "1-10,20", 3)
               # ('1', '10', '20')

               split(/(-|,)/, "1-10,20", 3)
               # ('1', '-', '10', ',', '20')

               split(/-|(,)/, "1-10,20", 3)
               # ('1', undef, '10', ',', '20')

               split(/(-)|,/, "1-10,20", 3)
               # ('1', '-', '10', undef, '20')

               split(/(-)|(,)/, "1-10,20", 3)
               # ('1', '-', undef, '10', undef, ',', '20')

       sprintf FORMAT, LIST
           Returns a string formatted by the usual "printf" conventions of the C library function
           "sprintf".  See below for more details and see sprintf(3) or printf(3) on your system
           for an explanation of the general principles.

           For example:

                   # Format number with up to 8 leading zeroes
                   my $result = sprintf("%08d", $number);

                   # Round number to 3 digits after decimal point
                   my $rounded = sprintf("%.3f", $number);

           Perl does its own "sprintf" formatting: it emulates the C function sprintf(3), but
           doesn't use it except for floating-point numbers, and even then only standard
           modifiers are allowed.  Non-standard extensions in your local sprintf(3) are therefore
           unavailable from Perl.

           Unlike "printf", "sprintf" does not do what you probably mean when you pass it an
           array as your first argument.  The array is given scalar context, and instead of using
           the 0th element of the array as the format, Perl will use the count of elements in the
           array as the format, which is almost never useful.

           Perl's "sprintf" permits the following universally-known conversions:

              %%    a percent sign
              %c    a character with the given number
              %s    a string
              %d    a signed integer, in decimal
              %u    an unsigned integer, in decimal
              %o    an unsigned integer, in octal
              %x    an unsigned integer, in hexadecimal
              %e    a floating-point number, in scientific notation
              %f    a floating-point number, in fixed decimal notation
              %g    a floating-point number, in %e or %f notation

           In addition, Perl permits the following widely-supported conversions:

              %X    like %x, but using upper-case letters
              %E    like %e, but using an upper-case "E"
              %G    like %g, but with an upper-case "E" (if applicable)
              %b    an unsigned integer, in binary
              %B    like %b, but using an upper-case "B" with the # flag
              %p    a pointer (outputs the Perl value's address in hexadecimal)
              %n    special: *stores* the number of characters output so far
                    into the next argument in the parameter list
              %a    hexadecimal floating point
              %A    like %a, but using upper-case letters

           Finally, for backward (and we do mean "backward") compatibility, Perl permits these
           unnecessary but widely-supported conversions:

              %i    a synonym for %d
              %D    a synonym for %ld
              %U    a synonym for %lu
              %O    a synonym for %lo
              %F    a synonym for %f

           Note that the number of exponent digits in the scientific notation produced by %e, %E,
           %g and %G for numbers with the modulus of the exponent less than 100 is system-
           dependent: it may be three or less (zero-padded as necessary).  In other words, 1.23
           times ten to the 99th may be either "1.23e99" or "1.23e099".  Similarly for %a and %A:
           the exponent or the hexadecimal digits may float: especially the "long doubles" Perl
           configuration option may cause surprises.

           Between the "%" and the format letter, you may specify several additional attributes
           controlling the interpretation of the format.  In order, these are:

           format parameter index
               An explicit format parameter index, such as "2$".  By default sprintf will format
               the next unused argument in the list, but this allows you to take the arguments
               out of order:

                 printf '%2$d %1$d', 12, 34;      # prints "34 12"
                 printf '%3$d %d %1$d', 1, 2, 3;  # prints "3 1 1"

           flags
               one or more of:

                  space   prefix non-negative number with a space
                  +       prefix non-negative number with a plus sign
                  -       left-justify within the field
                  0       use zeros, not spaces, to right-justify
                  #       ensure the leading "0" for any octal,
                          prefix non-zero hexadecimal with "0x" or "0X",
                          prefix non-zero binary with "0b" or "0B"

               For example:

                 printf '<% d>',  12;   # prints "< 12>"
                 printf '<% d>',   0;   # prints "< 0>"
                 printf '<% d>', -12;   # prints "<-12>"
                 printf '<%+d>',  12;   # prints "<+12>"
                 printf '<%+d>',   0;   # prints "<+0>"
                 printf '<%+d>', -12;   # prints "<-12>"
                 printf '<%6s>',  12;   # prints "<    12>"
                 printf '<%-6s>', 12;   # prints "<12    >"
                 printf '<%06s>', 12;   # prints "<000012>"
                 printf '<%#o>',  12;   # prints "<014>"
                 printf '<%#x>',  12;   # prints "<0xc>"
                 printf '<%#X>',  12;   # prints "<0XC>"
                 printf '<%#b>',  12;   # prints "<0b1100>"
                 printf '<%#B>',  12;   # prints "<0B1100>"

               When a space and a plus sign are given as the flags at once, the space is ignored.

                 printf '<%+ d>', 12;   # prints "<+12>"
                 printf '<% +d>', 12;   # prints "<+12>"

               When the # flag and a precision are given in the %o conversion, the precision is
               incremented if it's necessary for the leading "0".

                 printf '<%#.5o>', 012;      # prints "<00012>"
                 printf '<%#.5o>', 012345;   # prints "<012345>"
                 printf '<%#.0o>', 0;        # prints "<0>"

           vector flag
               This flag tells Perl to interpret the supplied string as a vector of integers, one
               for each character in the string.  Perl applies the format to each integer in
               turn, then joins the resulting strings with a separator (a dot "." by default).
               This can be useful for displaying ordinal values of characters in arbitrary
               strings:

                 printf "%vd", "AB\x{100}";           # prints "65.66.256"
                 printf "version is v%vd\n", $^V;     # Perl's version

               Put an asterisk "*" before the "v" to override the string to use to separate the
               numbers:

                 printf "address is %*vX\n", ":", $addr;   # IPv6 address
                 printf "bits are %0*v8b\n", " ", $bits;   # random bitstring

               You can also explicitly specify the argument number to use for the join string
               using something like "*2$v"; for example:

                 printf '%*4$vX %*4$vX %*4$vX',       # 3 IPv6 addresses
                         @addr[1..3], ":";

           (minimum) width
               Arguments are usually formatted to be only as wide as required to display the
               given value.  You can override the width by putting a number here, or get the
               width from the next argument (with "*") or from a specified argument (e.g., with
               "*2$"):

                printf "<%s>", "a";       # prints "<a>"
                printf "<%6s>", "a";      # prints "<     a>"
                printf "<%*s>", 6, "a";   # prints "<     a>"
                printf '<%*2$s>', "a", 6; # prints "<     a>"
                printf "<%2s>", "long";   # prints "<long>" (does not truncate)

               If a field width obtained through "*" is negative, it has the same effect as the
               "-" flag: left-justification.

           precision, or maximum width
               You can specify a precision (for numeric conversions) or a maximum width (for
               string conversions) by specifying a "." followed by a number.  For floating-point
               formats except "g" and "G", this specifies how many places right of the decimal
               point to show (the default being 6).  For example:

                 # these examples are subject to system-specific variation
                 printf '<%f>', 1;    # prints "<1.000000>"
                 printf '<%.1f>', 1;  # prints "<1.0>"
                 printf '<%.0f>', 1;  # prints "<1>"
                 printf '<%e>', 10;   # prints "<1.000000e+01>"
                 printf '<%.1e>', 10; # prints "<1.0e+01>"

               For "g" and "G", this specifies the maximum number of significant digits to show;
               for example:

                 # These examples are subject to system-specific variation.
                 printf '<%g>', 1;        # prints "<1>"
                 printf '<%.10g>', 1;     # prints "<1>"
                 printf '<%g>', 100;      # prints "<100>"
                 printf '<%.1g>', 100;    # prints "<1e+02>"
                 printf '<%.2g>', 100.01; # prints "<1e+02>"
                 printf '<%.5g>', 100.01; # prints "<100.01>"
                 printf '<%.4g>', 100.01; # prints "<100>"
                 printf '<%.1g>', 0.0111; # prints "<0.01>"
                 printf '<%.2g>', 0.0111; # prints "<0.011>"
                 printf '<%.3g>', 0.0111; # prints "<0.0111>"

               For integer conversions, specifying a precision implies that the output of the
               number itself should be zero-padded to this width, where the 0 flag is ignored:

                 printf '<%.6d>', 1;      # prints "<000001>"
                 printf '<%+.6d>', 1;     # prints "<+000001>"
                 printf '<%-10.6d>', 1;   # prints "<000001    >"
                 printf '<%10.6d>', 1;    # prints "<    000001>"
                 printf '<%010.6d>', 1;   # prints "<    000001>"
                 printf '<%+10.6d>', 1;   # prints "<   +000001>"

                 printf '<%.6x>', 1;      # prints "<000001>"
                 printf '<%#.6x>', 1;     # prints "<0x000001>"
                 printf '<%-10.6x>', 1;   # prints "<000001    >"
                 printf '<%10.6x>', 1;    # prints "<    000001>"
                 printf '<%010.6x>', 1;   # prints "<    000001>"
                 printf '<%#10.6x>', 1;   # prints "<  0x000001>"

               For string conversions, specifying a precision truncates the string to fit the
               specified width:

                 printf '<%.5s>', "truncated";   # prints "<trunc>"
                 printf '<%10.5s>', "truncated"; # prints "<     trunc>"

               You can also get the precision from the next argument using ".*", or from a
               specified argument (e.g., with ".*2$"):

                 printf '<%.6x>', 1;       # prints "<000001>"
                 printf '<%.*x>', 6, 1;    # prints "<000001>"

                 printf '<%.*2$x>', 1, 6;  # prints "<000001>"

                 printf '<%6.*2$x>', 1, 4; # prints "<  0001>"

               If a precision obtained through "*" is negative, it counts as having no precision
               at all.

                 printf '<%.*s>',  7, "string";   # prints "<string>"
                 printf '<%.*s>',  3, "string";   # prints "<str>"
                 printf '<%.*s>',  0, "string";   # prints "<>"
                 printf '<%.*s>', -1, "string";   # prints "<string>"

                 printf '<%.*d>',  1, 0;   # prints "<0>"
                 printf '<%.*d>',  0, 0;   # prints "<>"
                 printf '<%.*d>', -1, 0;   # prints "<0>"

           size
               For numeric conversions, you can specify the size to interpret the number as using
               "l", "h", "V", "q", "L", or "ll".  For integer conversions ("d u o x X b i D U
               O"), numbers are usually assumed to be whatever the default integer size is on
               your platform (usually 32 or 64 bits), but you can override this to use instead
               one of the standard C types, as supported by the compiler used to build Perl:

                  hh          interpret integer as C type "char" or "unsigned
                              char" on Perl 5.14 or later
                  h           interpret integer as C type "short" or
                              "unsigned short"
                  j           interpret integer as C type "intmax_t" on Perl
                              5.14 or later, and only with a C99 compiler
                              (unportable)
                  l           interpret integer as C type "long" or
                              "unsigned long"
                  q, L, or ll interpret integer as C type "long long",
                              "unsigned long long", or "quad" (typically
                              64-bit integers)
                  t           interpret integer as C type "ptrdiff_t" on Perl
                              5.14 or later
                  z           interpret integer as C type "size_t" on Perl 5.14
                              or later

               As of 5.14, none of these raises an exception if they are not supported on your
               platform.  However, if warnings are enabled, a warning of the "printf" warning
               class is issued on an unsupported conversion flag.  Should you instead prefer an
               exception, do this:

                   use warnings FATAL => "printf";

               If you would like to know about a version dependency before you start running the
               program, put something like this at its top:

                   use 5.014;  # for hh/j/t/z/ printf modifiers

               You can find out whether your Perl supports quads via Config:

                   use Config;
                   if ($Config{use64bitint} eq "define"
                       || $Config{longsize} >= 8) {
                       print "Nice quads!\n";
                   }

               For floating-point conversions ("e f g E F G"), numbers are usually assumed to be
               the default floating-point size on your platform (double or long double), but you
               can force "long double" with "q", "L", or "ll" if your platform supports them.
               You can find out whether your Perl supports long doubles via Config:

                   use Config;
                   print "long doubles\n" if $Config{d_longdbl} eq "define";

               You can find out whether Perl considers "long double" to be the default floating-
               point size to use on your platform via Config:

                   use Config;
                   if ($Config{uselongdouble} eq "define") {
                       print "long doubles by default\n";
                   }

               It can also be that long doubles and doubles are the same thing:

                       use Config;
                       ($Config{doublesize} == $Config{longdblsize}) &&
                               print "doubles are long doubles\n";

               The size specifier "V" has no effect for Perl code, but is supported for
               compatibility with XS code.  It means "use the standard size for a Perl integer or
               floating-point number", which is the default.

           order of arguments
               Normally, "sprintf" takes the next unused argument as the value to format for each
               format specification.  If the format specification uses "*" to require additional
               arguments, these are consumed from the argument list in the order they appear in
               the format specification before the value to format.  Where an argument is
               specified by an explicit index, this does not affect the normal order for the
               arguments, even when the explicitly specified index would have been the next
               argument.

               So:

                   printf "<%*.*s>", $a, $b, $c;

               uses $a for the width, $b for the precision, and $c as the value to format; while:

                 printf '<%*1$.*s>', $a, $b;

               would use $a for the width and precision, and $b as the value to format.

               Here are some more examples; be aware that when using an explicit index, the "$"
               may need escaping:

                printf "%2\$d %d\n",      12, 34;     # will print "34 12\n"
                printf "%2\$d %d %d\n",   12, 34;     # will print "34 12 34\n"
                printf "%3\$d %d %d\n",   12, 34, 56; # will print "56 12 34\n"
                printf "%2\$*3\$d %d\n",  12, 34,  3; # will print " 34 12\n"
                printf "%*1\$.*f\n",       4,  5, 10; # will print "5.0000\n"

           If "use locale" (including "use locale ':not_characters'") is in effect and
           "POSIX::setlocale" has been called, the character used for the decimal separator in
           formatted floating-point numbers is affected by the "LC_NUMERIC" locale.  See
           perllocale and POSIX.

       sqrt EXPR
       sqrt
           Return the positive square root of EXPR.  If EXPR is omitted, uses $_.  Works only for
           non-negative operands unless you've loaded the "Math::Complex" module.

               use Math::Complex;
               print sqrt(-4);    # prints 2i

       srand EXPR
       srand
           Sets and returns the random number seed for the "rand" operator.

           The point of the function is to "seed" the "rand" function so that "rand" can produce
           a different sequence each time you run your program.  When called with a parameter,
           "srand" uses that for the seed; otherwise it (semi-)randomly chooses a seed.  In
           either case, starting with Perl 5.14, it returns the seed.  To signal that your code
           will work only on Perls of a recent vintage:

               use 5.014;  # so srand returns the seed

           If "srand" is not called explicitly, it is called implicitly without a parameter at
           the first use of the "rand" operator.  However, there are a few situations where
           programs are likely to want to call "srand".  One is for generating predictable
           results, generally for testing or debugging.  There, you use "srand($seed)", with the
           same $seed each time.  Another case is that you may want to call "srand" after a
           "fork" to avoid child processes sharing the same seed value as the parent (and
           consequently each other).

           Do not call "srand()" (i.e., without an argument) more than once per process.  The
           internal state of the random number generator should contain more entropy than can be
           provided by any seed, so calling "srand" again actually loses randomness.

           Most implementations of "srand" take an integer and will silently truncate decimal
           numbers.  This means "srand(42)" will usually produce the same results as
           "srand(42.1)".  To be safe, always pass "srand" an integer.

           A typical use of the returned seed is for a test program which has too many
           combinations to test comprehensively in the time available to it each run.  It can
           test a random subset each time, and should there be a failure, log the seed used for
           that run so that it can later be used to reproduce the same results.

           "rand" is not cryptographically secure.  You should not rely on it in security-
           sensitive situations.  As of this writing, a number of third-party CPAN modules offer
           random number generators intended by their authors to be cryptographically secure,
           including: Data::Entropy, Crypt::Random, Math::Random::Secure, and Math::TrulyRandom.

       stat FILEHANDLE
       stat EXPR
       stat DIRHANDLE
       stat
           Returns a 13-element list giving the status info for a file, either the file opened
           via FILEHANDLE or DIRHANDLE, or named by EXPR.  If EXPR is omitted, it stats $_ (not
           "_"!).  Returns the empty list if "stat" fails.  Typically used as follows:

               my ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
                   $atime,$mtime,$ctime,$blksize,$blocks)
                      = stat($filename);

           Not all fields are supported on all filesystem types.  Here are the meanings of the
           fields:

             0 dev      device number of filesystem
             1 ino      inode number
             2 mode     file mode  (type and permissions)
             3 nlink    number of (hard) links to the file
             4 uid      numeric user ID of file's owner
             5 gid      numeric group ID of file's owner
             6 rdev     the device identifier (special files only)
             7 size     total size of file, in bytes
             8 atime    last access time in seconds since the epoch
             9 mtime    last modify time in seconds since the epoch
            10 ctime    inode change time in seconds since the epoch (*)
            11 blksize  preferred I/O size in bytes for interacting with the
                        file (may vary from file to file)
            12 blocks   actual number of system-specific blocks allocated
                        on disk (often, but not always, 512 bytes each)

           (The epoch was at 00:00 January 1, 1970 GMT.)

           (*) Not all fields are supported on all filesystem types.  Notably, the ctime field is
           non-portable.  In particular, you cannot expect it to be a "creation time"; see "Files
           and Filesystems" in perlport for details.

           If "stat" is passed the special filehandle consisting of an underline, no stat is
           done, but the current contents of the stat structure from the last "stat", "lstat", or
           filetest are returned.  Example:

               if (-x $file && (($d) = stat(_)) && $d < 0) {
                   print "$file is executable NFS file\n";
               }

           (This works on machines only for which the device number is negative under NFS.)

           Because the mode contains both the file type and its permissions, you should mask off
           the file type portion and (s)printf using a "%o" if you want to see the real
           permissions.

               my $mode = (stat($filename))[2];
               printf "Permissions are %04o\n", $mode & 07777;

           In scalar context, "stat" returns a boolean value indicating success or failure, and,
           if successful, sets the information associated with the special filehandle "_".

           The File::stat module provides a convenient, by-name access mechanism:

               use File::stat;
               my $sb = stat($filename);
               printf "File is %s, size is %s, perm %04o, mtime %s\n",
                      $filename, $sb->size, $sb->mode & 07777,
                      scalar localtime $sb->mtime;

           You can import symbolic mode constants ("S_IF*") and functions ("S_IS*") from the
           Fcntl module:

               use Fcntl ':mode';

               my $mode = (stat($filename))[2];

               my $user_rwx      = ($mode & S_IRWXU) >> 6;
               my $group_read    = ($mode & S_IRGRP) >> 3;
               my $other_execute =  $mode & S_IXOTH;

               printf "Permissions are %04o\n", S_IMODE($mode), "\n";

               my $is_setuid     =  $mode & S_ISUID;
               my $is_directory  =  S_ISDIR($mode);

           You could write the last two using the "-u" and "-d" operators.  Commonly available
           "S_IF*" constants are:

               # Permissions: read, write, execute, for user, group, others.

               S_IRWXU S_IRUSR S_IWUSR S_IXUSR
               S_IRWXG S_IRGRP S_IWGRP S_IXGRP
               S_IRWXO S_IROTH S_IWOTH S_IXOTH

               # Setuid/Setgid/Stickiness/SaveText.
               # Note that the exact meaning of these is system-dependent.

               S_ISUID S_ISGID S_ISVTX S_ISTXT

               # File types.  Not all are necessarily available on
               # your system.

               S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
               S_IFIFO S_IFSOCK S_IFWHT S_ENFMT

               # The following are compatibility aliases for S_IRUSR,
               # S_IWUSR, and S_IXUSR.

               S_IREAD S_IWRITE S_IEXEC

           and the "S_IF*" functions are

               S_IMODE($mode)    the part of $mode containing the permission
                                 bits and the setuid/setgid/sticky bits

               S_IFMT($mode)     the part of $mode containing the file type
                                 which can be bit-anded with (for example)
                                 S_IFREG or with the following functions

               # The operators -f, -d, -l, -b, -c, -p, and -S.

               S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
               S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)

               # No direct -X operator counterpart, but for the first one
               # the -g operator is often equivalent.  The ENFMT stands for
               # record flocking enforcement, a platform-dependent feature.

               S_ISENFMT($mode) S_ISWHT($mode)

           See your native chmod(2) and stat(2) documentation for more details about the "S_*"
           constants.  To get status info for a symbolic link instead of the target file behind
           the link, use the "lstat" function.

           Portability issues: "stat" in perlport.

       state VARLIST
       state TYPE VARLIST
       state VARLIST : ATTRS
       state TYPE VARLIST : ATTRS
           "state" declares a lexically scoped variable, just like "my".  However, those
           variables will never be reinitialized, contrary to lexical variables that are
           reinitialized each time their enclosing block is entered.  See "Persistent Private
           Variables" in perlsub for details.

           If more than one variable is listed, the list must be placed in parentheses.  With a
           parenthesised list, "undef" can be used as a dummy placeholder.  However, since
           initialization of state variables in list context is currently not possible this would
           serve no purpose.

           "state" is available only if the "state" feature is enabled or if it is prefixed with
           "CORE::".  The "state" feature is enabled automatically with a "use v5.10" (or higher)
           declaration in the current scope.

       study SCALAR
       study
           At this time, "study" does nothing. This may change in the future.

           Prior to Perl version 5.16, it would create an inverted index of all characters that
           occurred in the given SCALAR (or $_ if unspecified). When matching a pattern, the
           rarest character from the pattern would be looked up in this index. Rarity was based
           on some static frequency tables constructed from some C programs and English text.

       sub NAME BLOCK
       sub NAME (PROTO) BLOCK
       sub NAME : ATTRS BLOCK
       sub NAME (PROTO) : ATTRS BLOCK
           This is subroutine definition, not a real function per se.  Without a BLOCK it's just
           a forward declaration.  Without a NAME, it's an anonymous function declaration, so
           does return a value: the CODE ref of the closure just created.

           See perlsub and perlref for details about subroutines and references; see attributes
           and Attribute::Handlers for more information about attributes.

       __SUB__
           A special token that returns a reference to the current subroutine, or "undef" outside
           of a subroutine.

           The behaviour of "__SUB__" within a regex code block (such as "/(?{...})/") is subject
           to change.

           This token is only available under "use v5.16" or the "current_sub" feature.  See
           feature.

       substr EXPR,OFFSET,LENGTH,REPLACEMENT
       substr EXPR,OFFSET,LENGTH
       substr EXPR,OFFSET
           Extracts a substring out of EXPR and returns it.  First character is at offset zero.
           If OFFSET is negative, starts that far back from the end of the string.  If LENGTH is
           omitted, returns everything through the end of the string.  If LENGTH is negative,
           leaves that many characters off the end of the string.

               my $s = "The black cat climbed the green tree";
               my $color  = substr $s, 4, 5;      # black
               my $middle = substr $s, 4, -11;    # black cat climbed the
               my $end    = substr $s, 14;        # climbed the green tree
               my $tail   = substr $s, -4;        # tree
               my $z      = substr $s, -4, 2;     # tr

           You can use the "substr" function as an lvalue, in which case EXPR must itself be an
           lvalue.  If you assign something shorter than LENGTH, the string will shrink, and if
           you assign something longer than LENGTH, the string will grow to accommodate it.  To
           keep the string the same length, you may need to pad or chop your value using
           "sprintf".

           If OFFSET and LENGTH specify a substring that is partly outside the string, only the
           part within the string is returned.  If the substring is beyond either end of the
           string, "substr" returns the undefined value and produces a warning.  When used as an
           lvalue, specifying a substring that is entirely outside the string raises an
           exception.  Here's an example showing the behavior for boundary cases:

               my $name = 'fred';
               substr($name, 4) = 'dy';         # $name is now 'freddy'
               my $null = substr $name, 6, 2;   # returns "" (no warning)
               my $oops = substr $name, 7;      # returns undef, with warning
               substr($name, 7) = 'gap';        # raises an exception

           An alternative to using "substr" as an lvalue is to specify the replacement string as
           the 4th argument.  This allows you to replace parts of the EXPR and return what was
           there before in one operation, just as you can with "splice".

               my $s = "The black cat climbed the green tree";
               my $z = substr $s, 14, 7, "jumped from";    # climbed
               # $s is now "The black cat jumped from the green tree"

           Note that the lvalue returned by the three-argument version of "substr" acts as a
           'magic bullet'; each time it is assigned to, it remembers which part of the original
           string is being modified; for example:

               my $x = '1234';
               for (substr($x,1,2)) {
                   $_ = 'a';   print $x,"\n";    # prints 1a4
                   $_ = 'xyz'; print $x,"\n";    # prints 1xyz4
                   $x = '56789';
                   $_ = 'pq';  print $x,"\n";    # prints 5pq9
               }

           With negative offsets, it remembers its position from the end of the string when the
           target string is modified:

               my $x = '1234';
               for (substr($x, -3, 2)) {
                   $_ = 'a';   print $x,"\n";    # prints 1a4, as above
                   $x = 'abcdefg';
                   print $_,"\n";                # prints f
               }

           Prior to Perl version 5.10, the result of using an lvalue multiple times was
           unspecified.  Prior to 5.16, the result with negative offsets was unspecified.

       symlink OLDFILE,NEWFILE
           Creates a new filename symbolically linked to the old filename.  Returns 1 for
           success, 0 otherwise.  On systems that don't support symbolic links, raises an
           exception.  To check for that, use eval:

               my $symlink_exists = eval { symlink("",""); 1 };

           Portability issues: "symlink" in perlport.

       syscall NUMBER, LIST
           Calls the system call specified as the first element of the list, passing the
           remaining elements as arguments to the system call.  If unimplemented, raises an
           exception.  The arguments are interpreted as follows: if a given argument is numeric,
           the argument is passed as an int.  If not, the pointer to the string value is passed.
           You are responsible to make sure a string is pre-extended long enough to receive any
           result that might be written into a string.  You can't use a string literal (or other
           read-only string) as an argument to "syscall" because Perl has to assume that any
           string pointer might be written through.  If your integer arguments are not literals
           and have never been interpreted in a numeric context, you may need to add 0 to them to
           force them to look like numbers.  This emulates the "syswrite" function (or vice
           versa):

               require 'syscall.ph';        # may need to run h2ph
               my $s = "hi there\n";
               syscall(SYS_write(), fileno(STDOUT), $s, length $s);

           Note that Perl supports passing of up to only 14 arguments to your syscall, which in
           practice should (usually) suffice.

           Syscall returns whatever value returned by the system call it calls.  If the system
           call fails, "syscall" returns "-1" and sets $! (errno).  Note that some system calls
           can legitimately return "-1".  The proper way to handle such calls is to assign "$! =
           0" before the call, then check the value of $! if "syscall" returns "-1".

           There's a problem with "syscall(SYS_pipe())": it returns the file number of the read
           end of the pipe it creates, but there is no way to retrieve the file number of the
           other end.  You can avoid this problem by using "pipe" instead.

           Portability issues: "syscall" in perlport.

       sysopen FILEHANDLE,FILENAME,MODE
       sysopen FILEHANDLE,FILENAME,MODE,PERMS
           Opens the file whose filename is given by FILENAME, and associates it with FILEHANDLE.
           If FILEHANDLE is an expression, its value is used as the real filehandle wanted; an
           undefined scalar will be suitably autovivified.  This function calls the underlying
           operating system's open(2) function with the parameters FILENAME, MODE, and PERMS.

           Returns true on success and "undef" otherwise.

           The possible values and flag bits of the MODE parameter are system-dependent; they are
           available via the standard module "Fcntl".  See the documentation of your operating
           system's open(2) syscall to see which values and flag bits are available.  You may
           combine several flags using the "|"-operator.

           Some of the most common values are "O_RDONLY" for opening the file in read-only mode,
           "O_WRONLY" for opening the file in write-only mode, and "O_RDWR" for opening the file
           in read-write mode.

           For historical reasons, some values work on almost every system supported by Perl: 0
           means read-only, 1 means write-only, and 2 means read/write.  We know that these
           values do not work under OS/390 and on the Macintosh; you probably don't want to use
           them in new code.

           If the file named by FILENAME does not exist and the "open" call creates it (typically
           because MODE includes the "O_CREAT" flag), then the value of PERMS specifies the
           permissions of the newly created file.  If you omit the PERMS argument to "sysopen",
           Perl uses the octal value 0666.  These permission values need to be in octal, and are
           modified by your process's current "umask".

           In many systems the "O_EXCL" flag is available for opening files in exclusive mode.
           This is not locking: exclusiveness means here that if the file already exists,
           "sysopen" fails.  "O_EXCL" may not work on network filesystems, and has no effect
           unless the "O_CREAT" flag is set as well.  Setting "O_CREAT|O_EXCL" prevents the file
           from being opened if it is a symbolic link.  It does not protect against symbolic
           links in the file's path.

           Sometimes you may want to truncate an already-existing file.  This can be done using
           the "O_TRUNC" flag.  The behavior of "O_TRUNC" with "O_RDONLY" is undefined.

           You should seldom if ever use 0644 as argument to "sysopen", because that takes away
           the user's option to have a more permissive umask.  Better to omit it.  See "umask"
           for more on this.

           Note that under Perls older than 5.8.0, "sysopen" depends on the fdopen(3) C library
           function.  On many Unix systems, fdopen(3) is known to fail when file descriptors
           exceed a certain value, typically 255.  If you need more file descriptors than that,
           consider using the "POSIX::open" function.  For Perls 5.8.0 and later, PerlIO is (most
           often) the default.

           See perlopentut for a kinder, gentler explanation of opening files.

           Portability issues: "sysopen" in perlport.

       sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
       sysread FILEHANDLE,SCALAR,LENGTH
           Attempts to read LENGTH bytes of data into variable SCALAR from the specified
           FILEHANDLE, using read(2).  It bypasses buffered IO, so mixing this with other kinds
           of reads, "print", "write", "seek", "tell", or "eof" can cause confusion because the
           perlio or stdio layers usually buffer data.  Returns the number of bytes actually
           read, 0 at end of file, or undef if there was an error (in the latter case $! is also
           set).  SCALAR will be grown or shrunk so that the last byte actually read is the last
           byte of the scalar after the read.

           An OFFSET may be specified to place the read data at some place in the string other
           than the beginning.  A negative OFFSET specifies placement at that many characters
           counting backwards from the end of the string.  A positive OFFSET greater than the
           length of SCALAR results in the string being padded to the required size with "\0"
           bytes before the result of the read is appended.

           There is no syseof() function, which is ok, since "eof" doesn't work well on device
           files (like ttys) anyway.  Use "sysread" and check for a return value for 0 to decide
           whether you're done.

           Note that if the filehandle has been marked as ":utf8", Unicode characters are read
           instead of bytes (the LENGTH, OFFSET, and the return value of "sysread" are in Unicode
           characters).  The ":encoding(...)" layer implicitly introduces the ":utf8" layer.  See
           "binmode", "open", and the open pragma.

       sysseek FILEHANDLE,POSITION,WHENCE
           Sets FILEHANDLE's system position in bytes using lseek(2).  FILEHANDLE may be an
           expression whose value gives the name of the filehandle.  The values for WHENCE are 0
           to set the new position to POSITION; 1 to set the it to the current position plus
           POSITION; and 2 to set it to EOF plus POSITION, typically negative.

           Note the emphasis on bytes: even if the filehandle has been set to operate on
           characters (for example using the ":encoding(UTF-8)" I/O layer), the "seek", "tell",
           and "sysseek" family of functions use byte offsets, not character offsets, because
           seeking to a character offset would be very slow in a UTF-8 file.

           "sysseek" bypasses normal buffered IO, so mixing it with reads other than "sysread"
           (for example "readline" or "read"), "print", "write", "seek", "tell", or "eof" may
           cause confusion.

           For WHENCE, you may also use the constants "SEEK_SET", "SEEK_CUR", and "SEEK_END"
           (start of the file, current position, end of the file) from the Fcntl module.  Use of
           the constants is also more portable than relying on 0, 1, and 2.  For example to
           define a "systell" function:

               use Fcntl 'SEEK_CUR';
               sub systell { sysseek($_[0], 0, SEEK_CUR) }

           Returns the new position, or the undefined value on failure.  A position of zero is
           returned as the string "0 but true"; thus "sysseek" returns true on success and false
           on failure, yet you can still easily determine the new position.

       system LIST
       system PROGRAM LIST
           Does exactly the same thing as "exec", except that a fork is done first and the parent
           process waits for the child process to exit.  Note that argument processing varies
           depending on the number of arguments.  If there is more than one argument in LIST, or
           if LIST is an array with more than one value, starts the program given by the first
           element of the list with arguments given by the rest of the list.  If there is only
           one scalar argument, the argument is checked for shell metacharacters, and if there
           are any, the entire argument is passed to the system's command shell for parsing (this
           is "/bin/sh -c" on Unix platforms, but varies on other platforms).  If there are no
           shell metacharacters in the argument, it is split into words and passed directly to
           "execvp", which is more efficient.  On Windows, only the "system PROGRAM LIST" syntax
           will reliably avoid using the shell; "system LIST", even with more than one element,
           will fall back to the shell if the first spawn fails.

           Perl will attempt to flush all files opened for output before any operation that may
           do a fork, but this may not be supported on some platforms (see perlport).  To be
           safe, you may need to set $| ($AUTOFLUSH in English) or call the "autoflush" method of
           "IO::Handle" on any open handles.

           The return value is the exit status of the program as returned by the "wait" call.  To
           get the actual exit value, shift right by eight (see below).  See also "exec".  This
           is not what you want to use to capture the output from a command; for that you should
           use merely backticks or "qx//", as described in "`STRING`" in perlop.  Return value of
           -1 indicates a failure to start the program or an error of the wait(2) system call
           (inspect $! for the reason).

           If you'd like to make "system" (and many other bits of Perl) die on error, have a look
           at the autodie pragma.

           Like "exec", "system" allows you to lie to a program about its name if you use the
           "system PROGRAM LIST" syntax.  Again, see "exec".

           Since "SIGINT" and "SIGQUIT" are ignored during the execution of "system", if you
           expect your program to terminate on receipt of these signals you will need to arrange
           to do so yourself based on the return value.

               my @args = ("command", "arg1", "arg2");
               system(@args) == 0
                   or die "system @args failed: $?";

           If you'd like to manually inspect "system"'s failure, you can check all possible
           failure modes by inspecting $? like this:

               if ($? == -1) {
                   print "failed to execute: $!\n";
               }
               elsif ($? & 127) {
                   printf "child died with signal %d, %s coredump\n",
                       ($? & 127),  ($? & 128) ? 'with' : 'without';
               }
               else {
                   printf "child exited with value %d\n", $? >> 8;
               }

           Alternatively, you may inspect the value of "${^CHILD_ERROR_NATIVE}" with the "W*()"
           calls from the POSIX module.

           When "system"'s arguments are executed indirectly by the shell, results and return
           codes are subject to its quirks.  See "`STRING`" in perlop and "exec" for details.

           Since "system" does a "fork" and "wait" it may affect a "SIGCHLD" handler.  See
           perlipc for details.

           Portability issues: "system" in perlport.

       syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
       syswrite FILEHANDLE,SCALAR,LENGTH
       syswrite FILEHANDLE,SCALAR
           Attempts to write LENGTH bytes of data from variable SCALAR to the specified
           FILEHANDLE, using write(2).  If LENGTH is not specified, writes whole SCALAR.  It
           bypasses buffered IO, so mixing this with reads (other than "sysread)"), "print",
           "write", "seek", "tell", or "eof" may cause confusion because the perlio and stdio
           layers usually buffer data.  Returns the number of bytes actually written, or "undef"
           if there was an error (in this case the errno variable $! is also set).  If the LENGTH
           is greater than the data available in the SCALAR after the OFFSET, only as much data
           as is available will be written.

           An OFFSET may be specified to write the data from some part of the string other than
           the beginning.  A negative OFFSET specifies writing that many characters counting
           backwards from the end of the string.  If SCALAR is of length zero, you can only use
           an OFFSET of 0.

           WARNING: If the filehandle is marked ":utf8", Unicode characters encoded in UTF-8 are
           written instead of bytes, and the LENGTH, OFFSET, and return value of "syswrite" are
           in (UTF8-encoded Unicode) characters.  The ":encoding(...)" layer implicitly
           introduces the ":utf8" layer.  Alternately, if the handle is not marked with an
           encoding but you attempt to write characters with code points over 255, raises an
           exception.  See "binmode", "open", and the open pragma.

       tell FILEHANDLE
       tell
           Returns the current position in bytes for FILEHANDLE, or -1 on error.  FILEHANDLE may
           be an expression whose value gives the name of the actual filehandle.  If FILEHANDLE
           is omitted, assumes the file last read.

           Note the emphasis on bytes: even if the filehandle has been set to operate on
           characters (for example using the ":encoding(UTF-8)" I/O layer), the "seek", "tell",
           and "sysseek" family of functions use byte offsets, not character offsets, because
           seeking to a character offset would be very slow in a UTF-8 file.

           The return value of "tell" for the standard streams like the STDIN depends on the
           operating system: it may return -1 or something else.  "tell" on pipes, fifos, and
           sockets usually returns -1.

           There is no "systell" function.  Use "sysseek($fh, 0, 1)" for that.

           Do not use "tell" (or other buffered I/O operations) on a filehandle that has been
           manipulated by "sysread", "syswrite", or "sysseek".  Those functions ignore the
           buffering, while "tell" does not.

       telldir DIRHANDLE
           Returns the current position of the "readdir" routines on DIRHANDLE.  Value may be
           given to "seekdir" to access a particular location in a directory.  "telldir" has the
           same caveats about possible directory compaction as the corresponding system library
           routine.

       tie VARIABLE,CLASSNAME,LIST
           This function binds a variable to a package class that will provide the implementation
           for the variable.  VARIABLE is the name of the variable to be enchanted.  CLASSNAME is
           the name of a class implementing objects of correct type.  Any additional arguments
           are passed to the appropriate constructor method of the class (meaning "TIESCALAR",
           "TIEHANDLE", "TIEARRAY", or "TIEHASH").  Typically these are arguments such as might
           be passed to the dbm_open(3) function of C.  The object returned by the constructor is
           also returned by the "tie" function, which would be useful if you want to access other
           methods in CLASSNAME.

           Note that functions such as "keys" and "values" may return huge lists when used on
           large objects, like DBM files.  You may prefer to use the "each" function to iterate
           over such.  Example:

               # print out history file offsets
               use NDBM_File;
               tie(my %HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
               while (my ($key,$val) = each %HIST) {
                   print $key, ' = ', unpack('L', $val), "\n";
               }

           A class implementing a hash should have the following methods:

               TIEHASH classname, LIST
               FETCH this, key
               STORE this, key, value
               DELETE this, key
               CLEAR this
               EXISTS this, key
               FIRSTKEY this
               NEXTKEY this, lastkey
               SCALAR this
               DESTROY this
               UNTIE this

           A class implementing an ordinary array should have the following methods:

               TIEARRAY classname, LIST
               FETCH this, key
               STORE this, key, value
               FETCHSIZE this
               STORESIZE this, count
               CLEAR this
               PUSH this, LIST
               POP this
               SHIFT this
               UNSHIFT this, LIST
               SPLICE this, offset, length, LIST
               EXTEND this, count
               DELETE this, key
               EXISTS this, key
               DESTROY this
               UNTIE this

           A class implementing a filehandle should have the following methods:

               TIEHANDLE classname, LIST
               READ this, scalar, length, offset
               READLINE this
               GETC this
               WRITE this, scalar, length, offset
               PRINT this, LIST
               PRINTF this, format, LIST
               BINMODE this
               EOF this
               FILENO this
               SEEK this, position, whence
               TELL this
               OPEN this, mode, LIST
               CLOSE this
               DESTROY this
               UNTIE this

           A class implementing a scalar should have the following methods:

               TIESCALAR classname, LIST
               FETCH this,
               STORE this, value
               DESTROY this
               UNTIE this

           Not all methods indicated above need be implemented.  See perltie, Tie::Hash,
           Tie::Array, Tie::Scalar, and Tie::Handle.

           Unlike "dbmopen", the "tie" function will not "use" or "require" a module for you; you
           need to do that explicitly yourself.  See DB_File or the Config module for interesting
           "tie" implementations.

           For further details see perltie, "tied".

       tied VARIABLE
           Returns a reference to the object underlying VARIABLE (the same value that was
           originally returned by the "tie" call that bound the variable to a package.)  Returns
           the undefined value if VARIABLE isn't tied to a package.

       time
           Returns the number of non-leap seconds since whatever time the system considers to be
           the epoch, suitable for feeding to "gmtime" and "localtime".  On most systems the
           epoch is 00:00:00 UTC, January 1, 1970; a prominent exception being Mac OS Classic
           which uses 00:00:00, January 1, 1904 in the current local time zone for its epoch.

           For measuring time in better granularity than one second, use the Time::HiRes module
           from Perl 5.8 onwards (or from CPAN before then), or, if you have gettimeofday(2), you
           may be able to use the "syscall" interface of Perl.  See perlfaq8 for details.

           For date and time processing look at the many related modules on CPAN.  For a
           comprehensive date and time representation look at the DateTime module.

       times
           Returns a four-element list giving the user and system times in seconds for this
           process and any exited children of this process.

               my ($user,$system,$cuser,$csystem) = times;

           In scalar context, "times" returns $user.

           Children's times are only included for terminated children.

           Portability issues: "times" in perlport.

       tr///
           The transliteration operator.  Same as "y///".  See "Quote-Like Operators" in perlop.

       truncate FILEHANDLE,LENGTH
       truncate EXPR,LENGTH
           Truncates the file opened on FILEHANDLE, or named by EXPR, to the specified length.
           Raises an exception if truncate isn't implemented on your system.  Returns true if
           successful, "undef" on error.

           The behavior is undefined if LENGTH is greater than the length of the file.

           The position in the file of FILEHANDLE is left unchanged.  You may want to call seek
           before writing to the file.

           Portability issues: "truncate" in perlport.

       uc EXPR
       uc  Returns an uppercased version of EXPR.  This is the internal function implementing the
           "\U" escape in double-quoted strings.  It does not attempt to do titlecase mapping on
           initial letters.  See "ucfirst" for that.

           If EXPR is omitted, uses $_.

           This function behaves the same way under various pragmas, such as in a locale, as "lc"
           does.

       ucfirst EXPR
       ucfirst
           Returns the value of EXPR with the first character in uppercase (titlecase in
           Unicode).  This is the internal function implementing the "\u" escape in double-quoted
           strings.

           If EXPR is omitted, uses $_.

           This function behaves the same way under various pragmas, such as in a locale, as "lc"
           does.

       umask EXPR
       umask
           Sets the umask for the process to EXPR and returns the previous value.  If EXPR is
           omitted, merely returns the current umask.

           The Unix permission "rwxr-x---" is represented as three sets of three bits, or three
           octal digits: 0750 (the leading 0 indicates octal and isn't one of the digits).  The
           "umask" value is such a number representing disabled permissions bits.  The permission
           (or "mode") values you pass "mkdir" or "sysopen" are modified by your umask, so even
           if you tell "sysopen" to create a file with permissions 0777, if your umask is 0022,
           then the file will actually be created with permissions 0755.  If your "umask" were
           0027 (group can't write; others can't read, write, or execute), then passing "sysopen"
           0666 would create a file with mode 0640 (because "0666 &~ 027" is 0640).

           Here's some advice: supply a creation mode of 0666 for regular files (in "sysopen")
           and one of 0777 for directories (in "mkdir") and executable files.  This gives users
           the freedom of choice: if they want protected files, they might choose process umasks
           of 022, 027, or even the particularly antisocial mask of 077.  Programs should rarely
           if ever make policy decisions better left to the user.  The exception to this is when
           writing files that should be kept private: mail files, web browser cookies, .rhosts
           files, and so on.

           If umask(2) is not implemented on your system and you are trying to restrict access
           for yourself (i.e., "(EXPR & 0700) > 0"), raises an exception.  If umask(2) is not
           implemented and you are not trying to restrict access for yourself, returns "undef".

           Remember that a umask is a number, usually given in octal; it is not a string of octal
           digits.  See also "oct", if all you have is a string.

           Portability issues: "umask" in perlport.

       undef EXPR
       undef
           Undefines the value of EXPR, which must be an lvalue.  Use only on a scalar value, an
           array (using "@"), a hash (using "%"), a subroutine (using "&"), or a typeglob (using
           "*").  Saying "undef $hash{$key}" will probably not do what you expect on most
           predefined variables or DBM list values, so don't do that; see "delete".  Always
           returns the undefined value.  You can omit the EXPR, in which case nothing is
           undefined, but you still get an undefined value that you could, for instance, return
           from a subroutine, assign to a variable, or pass as a parameter.  Examples:

               undef $foo;
               undef $bar{'blurfl'};      # Compare to: delete $bar{'blurfl'};
               undef @ary;
               undef %hash;
               undef &mysub;
               undef *xyz;       # destroys $xyz, @xyz, %xyz, &xyz, etc.
               return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
               select undef, undef, undef, 0.25;
               my ($x, $y, undef, $z) = foo();    # Ignore third value returned

           Note that this is a unary operator, not a list operator.

       unlink LIST
       unlink
           Deletes a list of files.  On success, it returns the number of files it successfully
           deleted.  On failure, it returns false and sets $! (errno):

               my $unlinked = unlink 'a', 'b', 'c';
               unlink @goners;
               unlink glob "*.bak";

           On error, "unlink" will not tell you which files it could not remove.  If you want to
           know which files you could not remove, try them one at a time:

                foreach my $file ( @goners ) {
                    unlink $file or warn "Could not unlink $file: $!";
                }

           Note: "unlink" will not attempt to delete directories unless you are superuser and the
           -U flag is supplied to Perl.  Even if these conditions are met, be warned that
           unlinking a directory can inflict damage on your filesystem.  Finally, using "unlink"
           on directories is not supported on many operating systems.  Use "rmdir" instead.

           If LIST is omitted, "unlink" uses $_.

       unpack TEMPLATE,EXPR
       unpack TEMPLATE
           "unpack" does the reverse of "pack": it takes a string and expands it out into a list
           of values.  (In scalar context, it returns merely the first value produced.)

           If EXPR is omitted, unpacks the $_ string.  See perlpacktut for an introduction to
           this function.

           The string is broken into chunks described by the TEMPLATE.  Each chunk is converted
           separately to a value.  Typically, either the string is a result of "pack", or the
           characters of the string represent a C structure of some kind.

           The TEMPLATE has the same format as in the "pack" function.  Here's a subroutine that
           does substring:

               sub substr {
                   my ($what, $where, $howmuch) = @_;
                   unpack("x$where a$howmuch", $what);
               }

           and then there's

               sub ordinal { unpack("W",$_[0]); } # same as ord()

           In addition to fields allowed in "pack", you may prefix a field with a %<number> to
           indicate that you want a <number>-bit checksum of the items instead of the items
           themselves.  Default is a 16-bit checksum.  The checksum is calculated by summing
           numeric values of expanded values (for string fields the sum of "ord($char)" is taken;
           for bit fields the sum of zeroes and ones).

           For example, the following computes the same number as the System V sum program:

               my $checksum = do {
                   local $/;  # slurp!
                   unpack("%32W*", readline) % 65535;
               };

           The following efficiently counts the number of set bits in a bit vector:

               my $setbits = unpack("%32b*", $selectmask);

           The "p" and "P" formats should be used with care.  Since Perl has no way of checking
           whether the value passed to "unpack" corresponds to a valid memory location, passing a
           pointer value that's not known to be valid is likely to have disastrous consequences.

           If there are more pack codes or if the repeat count of a field or a group is larger
           than what the remainder of the input string allows, the result is not well defined:
           the repeat count may be decreased, or "unpack" may produce empty strings or zeros, or
           it may raise an exception.  If the input string is longer than one described by the
           TEMPLATE, the remainder of that input string is ignored.

           See "pack" for more examples and notes.

       unshift ARRAY,LIST
           Does the opposite of a "shift".  Or the opposite of a "push", depending on how you
           look at it.  Prepends list to the front of the array and returns the new number of
           elements in the array.

               unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;

           Note the LIST is prepended whole, not one element at a time, so the prepended elements
           stay in the same order.  Use "reverse" to do the reverse.

           Starting with Perl 5.14, an experimental feature allowed "unshift" to take a scalar
           expression. This experiment has been deemed unsuccessful, and was removed as of Perl
           5.24.

       untie VARIABLE
           Breaks the binding between a variable and a package.  (See tie.)  Has no effect if the
           variable is not tied.

       use Module VERSION LIST
       use Module VERSION
       use Module LIST
       use Module
       use VERSION
           Imports some semantics into the current package from the named module, generally by
           aliasing certain subroutine or variable names into your package.  It is exactly
           equivalent to

               BEGIN { require Module; Module->import( LIST ); }

           except that Module must be a bareword.  The importation can be made conditional by
           using the if module.

           In the peculiar "use VERSION" form, VERSION may be either a positive decimal fraction
           such as 5.006, which will be compared to $], or a v-string of the form v5.6.1, which
           will be compared to $^V (aka $PERL_VERSION).  An exception is raised if VERSION is
           greater than the version of the current Perl interpreter; Perl will not attempt to
           parse the rest of the file.  Compare with "require", which can do a similar check at
           run time.  Symmetrically, "no VERSION" allows you to specify that you want a version
           of Perl older than the specified one.

           Specifying VERSION as a literal of the form v5.6.1 should generally be avoided,
           because it leads to misleading error messages under earlier versions of Perl (that is,
           prior to 5.6.0) that do not support this syntax.  The equivalent numeric version
           should be used instead.

               use v5.6.1;     # compile time version check
               use 5.6.1;      # ditto
               use 5.006_001;  # ditto; preferred for backwards compatibility

           This is often useful if you need to check the current Perl version before "use"ing
           library modules that won't work with older versions of Perl.  (We try not to do this
           more than we have to.)

           "use VERSION" also lexically enables all features available in the requested version
           as defined by the feature pragma, disabling any features not in the requested
           version's feature bundle.  See feature.  Similarly, if the specified Perl version is
           greater than or equal to 5.12.0, strictures are enabled lexically as with "use
           strict".  Any explicit use of "use strict" or "no strict" overrides "use VERSION",
           even if it comes before it.  Later use of "use VERSION" will override all behavior of
           a previous "use VERSION", possibly removing the "strict" and "feature" added by "use
           VERSION".  "use VERSION" does not load the feature.pm or strict.pm files.

           The "BEGIN" forces the "require" and "import" to happen at compile time.  The
           "require" makes sure the module is loaded into memory if it hasn't been yet.  The
           "import" is not a builtin; it's just an ordinary static method call into the "Module"
           package to tell the module to import the list of features back into the current
           package.  The module can implement its "import" method any way it likes, though most
           modules just choose to derive their "import" method via inheritance from the
           "Exporter" class that is defined in the "Exporter" module.  See Exporter.  If no
           "import" method can be found, then the call is skipped, even if there is an AUTOLOAD
           method.

           If you do not want to call the package's "import" method (for instance, to stop your
           namespace from being altered), explicitly supply the empty list:

               use Module ();

           That is exactly equivalent to

               BEGIN { require Module }

           If the VERSION argument is present between Module and LIST, then the "use" will call
           the "VERSION" method in class Module with the given version as an argument:

               use Module 12.34;

           is equivalent to:

               BEGIN { require Module; Module->VERSION(12.34) }

           The default "VERSION" method, inherited from the "UNIVERSAL" class, croaks if the
           given version is larger than the value of the variable $Module::VERSION.

           Again, there is a distinction between omitting LIST ("import" called with no
           arguments) and an explicit empty LIST "()" ("import" not called).  Note that there is
           no comma after VERSION!

           Because this is a wide-open interface, pragmas (compiler directives) are also
           implemented this way.  Some of the currently implemented pragmas are:

               use constant;
               use diagnostics;
               use integer;
               use sigtrap  qw(SEGV BUS);
               use strict   qw(subs vars refs);
               use subs     qw(afunc blurfl);
               use warnings qw(all);
               use sort     qw(stable _quicksort _mergesort);

           Some of these pseudo-modules import semantics into the current block scope (like
           "strict" or "integer", unlike ordinary modules, which import symbols into the current
           package (which are effective through the end of the file).

           Because "use" takes effect at compile time, it doesn't respect the ordinary flow
           control of the code being compiled.  In particular, putting a "use" inside the false
           branch of a conditional doesn't prevent it from being processed.  If a module or
           pragma only needs to be loaded conditionally, this can be done using the if pragma:

               use if $] < 5.008, "utf8";
               use if WANT_WARNINGS, warnings => qw(all);

           There's a corresponding "no" declaration that unimports meanings imported by "use",
           i.e., it calls "Module->unimport(LIST)" instead of "import".  It behaves just as
           "import" does with VERSION, an omitted or empty LIST, or no unimport method being
           found.

               no integer;
               no strict 'refs';
               no warnings;

           Care should be taken when using the "no VERSION" form of "no".  It is only meant to be
           used to assert that the running Perl is of a earlier version than its argument and not
           to undo the feature-enabling side effects of "use VERSION".

           See perlmodlib for a list of standard modules and pragmas.  See perlrun for the "-M"
           and "-m" command-line options to Perl that give "use" functionality from the command-
           line.

       utime LIST
           Changes the access and modification times on each file of a list of files.  The first
           two elements of the list must be the NUMERIC access and modification times, in that
           order.  Returns the number of files successfully changed.  The inode change time of
           each file is set to the current time.  For example, this code has the same effect as
           the Unix touch(1) command when the files already exist and belong to the user running
           the program:

               #!/usr/bin/perl
               my $atime = my $mtime = time;
               utime $atime, $mtime, @ARGV;

           Since Perl 5.8.0, if the first two elements of the list are "undef", the utime(2)
           syscall from your C library is called with a null second argument.  On most systems,
           this will set the file's access and modification times to the current time (i.e.,
           equivalent to the example above) and will work even on files you don't own provided
           you have write permission:

               for my $file (@ARGV) {
                   utime(undef, undef, $file)
                       || warn "Couldn't touch $file: $!";
               }

           Under NFS this will use the time of the NFS server, not the time of the local machine.
           If there is a time synchronization problem, the NFS server and local machine will have
           different times.  The Unix touch(1) command will in fact normally use this form
           instead of the one shown in the first example.

           Passing only one of the first two elements as "undef" is equivalent to passing a 0 and
           will not have the effect described when both are "undef".  This also triggers an
           uninitialized warning.

           On systems that support futimes(2), you may pass filehandles among the files.  On
           systems that don't support futimes(2), passing filehandles raises an exception.
           Filehandles must be passed as globs or glob references to be recognized; barewords are
           considered filenames.

           Portability issues: "utime" in perlport.

       values HASH
       values ARRAY
           In list context, returns a list consisting of all the values of the named hash.  In
           Perl 5.12 or later only, will also return a list of the values of an array; prior to
           that release, attempting to use an array argument will produce a syntax error.  In
           scalar context, returns the number of values.

           Hash entries are returned in an apparently random order.  The actual random order is
           specific to a given hash; the exact same series of operations on two hashes may result
           in a different order for each hash.  Any insertion into the hash may change the order,
           as will any deletion, with the exception that the most recent key returned by "each"
           or "keys" may be deleted without changing the order.  So long as a given hash is
           unmodified you may rely on "keys", "values" and "each" to repeatedly return the same
           order as each other.  See "Algorithmic Complexity Attacks" in perlsec for details on
           why hash order is randomized.  Aside from the guarantees provided here the exact
           details of Perl's hash algorithm and the hash traversal order are subject to change in
           any release of Perl.  Tied hashes may behave differently to Perl's hashes with respect
           to changes in order on insertion and deletion of items.

           As a side effect, calling "values" resets the HASH or ARRAY's internal iterator, see
           "each".  (In particular, calling "values" in void context resets the iterator with no
           other overhead.  Apart from resetting the iterator, "values @array" in list context is
           the same as plain @array.  (We recommend that you use void context "keys @array" for
           this, but reasoned that taking "values @array" out would require more documentation
           than leaving it in.)

           Note that the values are not copied, which means modifying them will modify the
           contents of the hash:

               for (values %hash)      { s/foo/bar/g }  # modifies %hash values
               for (@hash{keys %hash}) { s/foo/bar/g }  # same

           Starting with Perl 5.14, an experimental feature allowed "values" to take a scalar
           expression. This experiment has been deemed unsuccessful, and was removed as of Perl
           5.24.

           To avoid confusing would-be users of your code who are running earlier versions of
           Perl with mysterious syntax errors, put this sort of thing at the top of your file to
           signal that your code will work only on Perls of a recent vintage:

               use 5.012;  # so keys/values/each work on arrays

           See also "keys", "each", and "sort".

       vec EXPR,OFFSET,BITS
           Treats the string in EXPR as a bit vector made up of elements of width BITS and
           returns the value of the element specified by OFFSET as an unsigned integer.  BITS
           therefore specifies the number of bits that are reserved for each element in the bit
           vector.  This must be a power of two from 1 to 32 (or 64, if your platform supports
           that).

           If BITS is 8, "elements" coincide with bytes of the input string.

           If BITS is 16 or more, bytes of the input string are grouped into chunks of size
           BITS/8, and each group is converted to a number as with "pack"/"unpack" with big-
           endian formats "n"/"N" (and analogously for BITS==64).  See "pack" for details.

           If bits is 4 or less, the string is broken into bytes, then the bits of each byte are
           broken into 8/BITS groups.  Bits of a byte are numbered in a little-endian-ish way, as
           in 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80.  For example, breaking the single
           input byte "chr(0x36)" into two groups gives a list "(0x6, 0x3)"; breaking it into 4
           groups gives "(0x2, 0x1, 0x3, 0x0)".

           "vec" may also be assigned to, in which case parentheses are needed to give the
           expression the correct precedence as in

               vec($image, $max_x * $x + $y, 8) = 3;

           If the selected element is outside the string, the value 0 is returned.  If an element
           off the end of the string is written to, Perl will first extend the string with
           sufficiently many zero bytes.   It is an error to try to write off the beginning of
           the string (i.e., negative OFFSET).

           If the string happens to be encoded as UTF-8 internally (and thus has the UTF8 flag
           set), "vec" tries to convert it to use a one-byte-per-character internal
           representation. However, if the string contains characters with values of 256 or
           higher, that conversion will fail. In that situation, "vec" will operate on the
           underlying buffer regardless, in its internal UTF-8 representation.

           Strings created with "vec" can also be manipulated with the logical operators "|",
           "&", "^", and "~".  These operators will assume a bit vector operation is desired when
           both operands are strings.  See "Bitwise String Operators" in perlop.

           The following code will build up an ASCII string saying 'PerlPerlPerl'.  The comments
           show the string after each step.  Note that this code works in the same way on big-
           endian or little-endian machines.

               my $foo = '';
               vec($foo,  0, 32) = 0x5065726C; # 'Perl'

               # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
               print vec($foo, 0, 8);  # prints 80 == 0x50 == ord('P')

               vec($foo,  2, 16) = 0x5065; # 'PerlPe'
               vec($foo,  3, 16) = 0x726C; # 'PerlPerl'
               vec($foo,  8,  8) = 0x50;   # 'PerlPerlP'
               vec($foo,  9,  8) = 0x65;   # 'PerlPerlPe'
               vec($foo, 20,  4) = 2;      # 'PerlPerlPe'   . "\x02"
               vec($foo, 21,  4) = 7;      # 'PerlPerlPer'
                                              # 'r' is "\x72"
               vec($foo, 45,  2) = 3;      # 'PerlPerlPer'  . "\x0c"
               vec($foo, 93,  1) = 1;      # 'PerlPerlPer'  . "\x2c"
               vec($foo, 94,  1) = 1;      # 'PerlPerlPerl'
                                              # 'l' is "\x6c"

           To transform a bit vector into a string or list of 0's and 1's, use these:

               my $bits = unpack("b*", $vector);
               my @bits = split(//, unpack("b*", $vector));

           If you know the exact length in bits, it can be used in place of the "*".

           Here is an example to illustrate how the bits actually fall in place:

             #!/usr/bin/perl -wl

             print <<'EOT';
                                               0         1         2         3
                                unpack("V",$_) 01234567890123456789012345678901
             ------------------------------------------------------------------
             EOT

             for $w (0..3) {
                 $width = 2**$w;
                 for ($shift=0; $shift < $width; ++$shift) {
                     for ($off=0; $off < 32/$width; ++$off) {
                         $str = pack("B*", "0"x32);
                         $bits = (1<<$shift);
                         vec($str, $off, $width) = $bits;
                         $res = unpack("b*",$str);
                         $val = unpack("V", $str);
                         write;
                     }
                 }
             }

             format STDOUT =
             vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
             $off, $width, $bits, $val, $res
             .
             __END__

           Regardless of the machine architecture on which it runs, the example above should
           print the following table:

                                               0         1         2         3
                                unpack("V",$_) 01234567890123456789012345678901
             ------------------------------------------------------------------
             vec($_, 0, 1) = 1   ==          1 10000000000000000000000000000000
             vec($_, 1, 1) = 1   ==          2 01000000000000000000000000000000
             vec($_, 2, 1) = 1   ==          4 00100000000000000000000000000000
             vec($_, 3, 1) = 1   ==          8 00010000000000000000000000000000
             vec($_, 4, 1) = 1   ==         16 00001000000000000000000000000000
             vec($_, 5, 1) = 1   ==         32 00000100000000000000000000000000
             vec($_, 6, 1) = 1   ==         64 00000010000000000000000000000000
             vec($_, 7, 1) = 1   ==        128 00000001000000000000000000000000
             vec($_, 8, 1) = 1   ==        256 00000000100000000000000000000000
             vec($_, 9, 1) = 1   ==        512 00000000010000000000000000000000
             vec($_,10, 1) = 1   ==       1024 00000000001000000000000000000000
             vec($_,11, 1) = 1   ==       2048 00000000000100000000000000000000
             vec($_,12, 1) = 1   ==       4096 00000000000010000000000000000000
             vec($_,13, 1) = 1   ==       8192 00000000000001000000000000000000
             vec($_,14, 1) = 1   ==      16384 00000000000000100000000000000000
             vec($_,15, 1) = 1   ==      32768 00000000000000010000000000000000
             vec($_,16, 1) = 1   ==      65536 00000000000000001000000000000000
             vec($_,17, 1) = 1   ==     131072 00000000000000000100000000000000
             vec($_,18, 1) = 1   ==     262144 00000000000000000010000000000000
             vec($_,19, 1) = 1   ==     524288 00000000000000000001000000000000
             vec($_,20, 1) = 1   ==    1048576 00000000000000000000100000000000
             vec($_,21, 1) = 1   ==    2097152 00000000000000000000010000000000
             vec($_,22, 1) = 1   ==    4194304 00000000000000000000001000000000
             vec($_,23, 1) = 1   ==    8388608 00000000000000000000000100000000
             vec($_,24, 1) = 1   ==   16777216 00000000000000000000000010000000
             vec($_,25, 1) = 1   ==   33554432 00000000000000000000000001000000
             vec($_,26, 1) = 1   ==   67108864 00000000000000000000000000100000
             vec($_,27, 1) = 1   ==  134217728 00000000000000000000000000010000
             vec($_,28, 1) = 1   ==  268435456 00000000000000000000000000001000
             vec($_,29, 1) = 1   ==  536870912 00000000000000000000000000000100
             vec($_,30, 1) = 1   == 1073741824 00000000000000000000000000000010
             vec($_,31, 1) = 1   == 2147483648 00000000000000000000000000000001
             vec($_, 0, 2) = 1   ==          1 10000000000000000000000000000000
             vec($_, 1, 2) = 1   ==          4 00100000000000000000000000000000
             vec($_, 2, 2) = 1   ==         16 00001000000000000000000000000000
             vec($_, 3, 2) = 1   ==         64 00000010000000000000000000000000
             vec($_, 4, 2) = 1   ==        256 00000000100000000000000000000000
             vec($_, 5, 2) = 1   ==       1024 00000000001000000000000000000000
             vec($_, 6, 2) = 1   ==       4096 00000000000010000000000000000000
             vec($_, 7, 2) = 1   ==      16384 00000000000000100000000000000000
             vec($_, 8, 2) = 1   ==      65536 00000000000000001000000000000000
             vec($_, 9, 2) = 1   ==     262144 00000000000000000010000000000000
             vec($_,10, 2) = 1   ==    1048576 00000000000000000000100000000000
             vec($_,11, 2) = 1   ==    4194304 00000000000000000000001000000000
             vec($_,12, 2) = 1   ==   16777216 00000000000000000000000010000000
             vec($_,13, 2) = 1   ==   67108864 00000000000000000000000000100000
             vec($_,14, 2) = 1   ==  268435456 00000000000000000000000000001000
             vec($_,15, 2) = 1   == 1073741824 00000000000000000000000000000010
             vec($_, 0, 2) = 2   ==          2 01000000000000000000000000000000
             vec($_, 1, 2) = 2   ==          8 00010000000000000000000000000000
             vec($_, 2, 2) = 2   ==         32 00000100000000000000000000000000
             vec($_, 3, 2) = 2   ==        128 00000001000000000000000000000000
             vec($_, 4, 2) = 2   ==        512 00000000010000000000000000000000
             vec($_, 5, 2) = 2   ==       2048 00000000000100000000000000000000
             vec($_, 6, 2) = 2   ==       8192 00000000000001000000000000000000
             vec($_, 7, 2) = 2   ==      32768 00000000000000010000000000000000
             vec($_, 8, 2) = 2   ==     131072 00000000000000000100000000000000
             vec($_, 9, 2) = 2   ==     524288 00000000000000000001000000000000
             vec($_,10, 2) = 2   ==    2097152 00000000000000000000010000000000
             vec($_,11, 2) = 2   ==    8388608 00000000000000000000000100000000
             vec($_,12, 2) = 2   ==   33554432 00000000000000000000000001000000
             vec($_,13, 2) = 2   ==  134217728 00000000000000000000000000010000
             vec($_,14, 2) = 2   ==  536870912 00000000000000000000000000000100
             vec($_,15, 2) = 2   == 2147483648 00000000000000000000000000000001
             vec($_, 0, 4) = 1   ==          1 10000000000000000000000000000000
             vec($_, 1, 4) = 1   ==         16 00001000000000000000000000000000
             vec($_, 2, 4) = 1   ==        256 00000000100000000000000000000000
             vec($_, 3, 4) = 1   ==       4096 00000000000010000000000000000000
             vec($_, 4, 4) = 1   ==      65536 00000000000000001000000000000000
             vec($_, 5, 4) = 1   ==    1048576 00000000000000000000100000000000
             vec($_, 6, 4) = 1   ==   16777216 00000000000000000000000010000000
             vec($_, 7, 4) = 1   ==  268435456 00000000000000000000000000001000
             vec($_, 0, 4) = 2   ==          2 01000000000000000000000000000000
             vec($_, 1, 4) = 2   ==         32 00000100000000000000000000000000
             vec($_, 2, 4) = 2   ==        512 00000000010000000000000000000000
             vec($_, 3, 4) = 2   ==       8192 00000000000001000000000000000000
             vec($_, 4, 4) = 2   ==     131072 00000000000000000100000000000000
             vec($_, 5, 4) = 2   ==    2097152 00000000000000000000010000000000
             vec($_, 6, 4) = 2   ==   33554432 00000000000000000000000001000000
             vec($_, 7, 4) = 2   ==  536870912 00000000000000000000000000000100
             vec($_, 0, 4) = 4   ==          4 00100000000000000000000000000000
             vec($_, 1, 4) = 4   ==         64 00000010000000000000000000000000
             vec($_, 2, 4) = 4   ==       1024 00000000001000000000000000000000
             vec($_, 3, 4) = 4   ==      16384 00000000000000100000000000000000
             vec($_, 4, 4) = 4   ==     262144 00000000000000000010000000000000
             vec($_, 5, 4) = 4   ==    4194304 00000000000000000000001000000000
             vec($_, 6, 4) = 4   ==   67108864 00000000000000000000000000100000
             vec($_, 7, 4) = 4   == 1073741824 00000000000000000000000000000010
             vec($_, 0, 4) = 8   ==          8 00010000000000000000000000000000
             vec($_, 1, 4) = 8   ==        128 00000001000000000000000000000000
             vec($_, 2, 4) = 8   ==       2048 00000000000100000000000000000000
             vec($_, 3, 4) = 8   ==      32768 00000000000000010000000000000000
             vec($_, 4, 4) = 8   ==     524288 00000000000000000001000000000000
             vec($_, 5, 4) = 8   ==    8388608 00000000000000000000000100000000
             vec($_, 6, 4) = 8   ==  134217728 00000000000000000000000000010000
             vec($_, 7, 4) = 8   == 2147483648 00000000000000000000000000000001
             vec($_, 0, 8) = 1   ==          1 10000000000000000000000000000000
             vec($_, 1, 8) = 1   ==        256 00000000100000000000000000000000
             vec($_, 2, 8) = 1   ==      65536 00000000000000001000000000000000
             vec($_, 3, 8) = 1   ==   16777216 00000000000000000000000010000000
             vec($_, 0, 8) = 2   ==          2 01000000000000000000000000000000
             vec($_, 1, 8) = 2   ==        512 00000000010000000000000000000000
             vec($_, 2, 8) = 2   ==     131072 00000000000000000100000000000000
             vec($_, 3, 8) = 2   ==   33554432 00000000000000000000000001000000
             vec($_, 0, 8) = 4   ==          4 00100000000000000000000000000000
             vec($_, 1, 8) = 4   ==       1024 00000000001000000000000000000000
             vec($_, 2, 8) = 4   ==     262144 00000000000000000010000000000000
             vec($_, 3, 8) = 4   ==   67108864 00000000000000000000000000100000
             vec($_, 0, 8) = 8   ==          8 00010000000000000000000000000000
             vec($_, 1, 8) = 8   ==       2048 00000000000100000000000000000000
             vec($_, 2, 8) = 8   ==     524288 00000000000000000001000000000000
             vec($_, 3, 8) = 8   ==  134217728 00000000000000000000000000010000
             vec($_, 0, 8) = 16  ==         16 00001000000000000000000000000000
             vec($_, 1, 8) = 16  ==       4096 00000000000010000000000000000000
             vec($_, 2, 8) = 16  ==    1048576 00000000000000000000100000000000
             vec($_, 3, 8) = 16  ==  268435456 00000000000000000000000000001000
             vec($_, 0, 8) = 32  ==         32 00000100000000000000000000000000
             vec($_, 1, 8) = 32  ==       8192 00000000000001000000000000000000
             vec($_, 2, 8) = 32  ==    2097152 00000000000000000000010000000000
             vec($_, 3, 8) = 32  ==  536870912 00000000000000000000000000000100
             vec($_, 0, 8) = 64  ==         64 00000010000000000000000000000000
             vec($_, 1, 8) = 64  ==      16384 00000000000000100000000000000000
             vec($_, 2, 8) = 64  ==    4194304 00000000000000000000001000000000
             vec($_, 3, 8) = 64  == 1073741824 00000000000000000000000000000010
             vec($_, 0, 8) = 128 ==        128 00000001000000000000000000000000
             vec($_, 1, 8) = 128 ==      32768 00000000000000010000000000000000
             vec($_, 2, 8) = 128 ==    8388608 00000000000000000000000100000000
             vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001

       wait
           Behaves like wait(2) on your system: it waits for a child process to terminate and
           returns the pid of the deceased process, or "-1" if there are no child processes.  The
           status is returned in $? and "${^CHILD_ERROR_NATIVE}".  Note that a return value of
           "-1" could mean that child processes are being automatically reaped, as described in
           perlipc.

           If you use "wait" in your handler for $SIG{CHLD}, it may accidentally wait for the
           child created by "qx" or "system".  See perlipc for details.

           Portability issues: "wait" in perlport.

       waitpid PID,FLAGS
           Waits for a particular child process to terminate and returns the pid of the deceased
           process, or "-1" if there is no such child process.  A non-blocking wait (with WNOHANG
           in FLAGS) can return 0 if there are child processes matching PID but none have
           terminated yet.  The status is returned in $? and "${^CHILD_ERROR_NATIVE}".

           A PID of 0 indicates to wait for any child process whose process group ID is equal to
           that of the current process.  A PID of less than "-1" indicates to wait for any child
           process whose process group ID is equal to -PID.  A PID of "-1" indicates to wait for
           any child process.

           If you say

               use POSIX ":sys_wait_h";

               my $kid;
               do {
                   $kid = waitpid(-1, WNOHANG);
               } while $kid > 0;

           or

               1 while waitpid(-1, WNOHANG) > 0;

           then you can do a non-blocking wait for all pending zombie processes (see "WAIT" in
           POSIX).  Non-blocking wait is available on machines supporting either the waitpid(2)
           or wait4(2) syscalls.  However, waiting for a particular pid with FLAGS of 0 is
           implemented everywhere.  (Perl emulates the system call by remembering the status
           values of processes that have exited but have not been harvested by the Perl script
           yet.)

           Note that on some systems, a return value of "-1" could mean that child processes are
           being automatically reaped.  See perlipc for details, and for other examples.

           Portability issues: "waitpid" in perlport.

       wantarray
           Returns true if the context of the currently executing subroutine or "eval" is looking
           for a list value.  Returns false if the context is looking for a scalar.  Returns the
           undefined value if the context is looking for no value (void context).

               return unless defined wantarray; # don't bother doing more
               my @a = complex_calculation();
               return wantarray ? @a : "@a";

           "wantarray"'s result is unspecified in the top level of a file, in a "BEGIN",
           "UNITCHECK", "CHECK", "INIT" or "END" block, or in a "DESTROY" method.

           This function should have been named wantlist() instead.

       warn LIST
           Prints the value of LIST to STDERR.  If the last element of LIST does not end in a
           newline, it appends the same file/line number text as "die" does.

           If the output is empty and $@ already contains a value (typically from a previous
           eval) that value is used after appending "\t...caught" to $@.  This is useful for
           staying almost, but not entirely similar to "die".

           If $@ is empty, then the string "Warning: Something's wrong" is used.

           No message is printed if there is a $SIG{__WARN__} handler installed.  It is the
           handler's responsibility to deal with the message as it sees fit (like, for instance,
           converting it into a "die").  Most handlers must therefore arrange to actually display
           the warnings that they are not prepared to deal with, by calling "warn" again in the
           handler.  Note that this is quite safe and will not produce an endless loop, since
           "__WARN__" hooks are not called from inside one.

           You will find this behavior is slightly different from that of $SIG{__DIE__} handlers
           (which don't suppress the error text, but can instead call "die" again to change it).

           Using a "__WARN__" handler provides a powerful way to silence all warnings (even the
           so-called mandatory ones).  An example:

               # wipe out *all* compile-time warnings
               BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
               my $foo = 10;
               my $foo = 20;          # no warning about duplicate my $foo,
                                      # but hey, you asked for it!
               # no compile-time or run-time warnings before here
               $DOWARN = 1;

               # run-time warnings enabled after here
               warn "\$foo is alive and $foo!";     # does show up

           See perlvar for details on setting %SIG entries and for more examples.  See the Carp
           module for other kinds of warnings using its "carp" and "cluck" functions.

       write FILEHANDLE
       write EXPR
       write
           Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, using the
           format associated with that file.  By default the format for a file is the one having
           the same name as the filehandle, but the format for the current output channel (see
           the "select" function) may be set explicitly by assigning the name of the format to
           the $~ variable.

           Top of form processing is handled automatically:  if there is insufficient room on the
           current page for the formatted record, the page is advanced by writing a form feed and
           a special top-of-page format is used to format the new page header before the record
           is written.  By default, the top-of-page format is the name of the filehandle with
           "_TOP" appended, or "top" in the current package if the former does not exist.  This
           would be a problem with autovivified filehandles, but it may be dynamically set to the
           format of your choice by assigning the name to the $^ variable while that filehandle
           is selected.  The number of lines remaining on the current page is in variable "$-",
           which can be set to 0 to force a new page.

           If FILEHANDLE is unspecified, output goes to the current default output channel, which
           starts out as STDOUT but may be changed by the "select" operator.  If the FILEHANDLE
           is an EXPR, then the expression is evaluated and the resulting string is used to look
           up the name of the FILEHANDLE at run time.  For more on formats, see perlform.

           Note that write is not the opposite of "read".  Unfortunately.

       y///
           The transliteration operator.  Same as "tr///".  See "Quote-Like Operators" in perlop.

   Non-function Keywords by Cross-reference
       perldata

       __DATA__
       __END__
           These keywords are documented in "Special Literals" in perldata.

       perlmod

       BEGIN
       CHECK
       END
       INIT
       UNITCHECK
           These compile phase keywords are documented in "BEGIN, UNITCHECK, CHECK, INIT and END"
           in perlmod.

       perlobj

       DESTROY
           This method keyword is documented in "Destructors" in perlobj.

       perlop

       and
       cmp
       eq
       ge
       gt
       le
       lt
       ne
       not
       or
       x
       xor These operators are documented in perlop.

       perlsub

       AUTOLOAD
           This keyword is documented in "Autoloading" in perlsub.

       perlsyn

       else
       elsif
       for
       foreach
       if
       unless
       until
       while
           These flow-control keywords are documented in "Compound Statements" in perlsyn.

       elseif
           The "else if" keyword is spelled "elsif" in Perl.  There's no "elif" or "else if"
           either.  It does parse "elseif", but only to warn you about not using it.

           See the documentation for flow-control keywords in "Compound Statements" in perlsyn.

       default
       given
       when
           These flow-control keywords related to the experimental switch feature are documented
           in "Switch Statements" in perlsyn.