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       perldebug - Perl debugging


       First of all, have you tried using "use strict;" and "use warnings;"?

       If you're new to the Perl debugger, you may prefer to read perldebtut, which is a tutorial
       introduction to the debugger.

       If you're looking for the nitty gritty details of how the debugger is implemented, you may
       prefer to read perldebguts.

       For in-depth technical usage details, see, the documentation of the debugger

The Perl Debugger

       If you invoke Perl with the -d switch, your script runs under the Perl source debugger.
       This works like an interactive Perl environment, prompting for debugger commands that let
       you examine source code, set breakpoints, get stack backtraces, change the values of
       variables, etc.  This is so convenient that you often fire up the debugger all by itself
       just to test out Perl constructs interactively to see what they do.  For example:

           $ perl -d -e 42

       In Perl, the debugger is not a separate program the way it usually is in the typical
       compiled environment.  Instead, the -d flag tells the compiler to insert source
       information into the parse trees it's about to hand off to the interpreter.  That means
       your code must first compile correctly for the debugger to work on it.  Then when the
       interpreter starts up, it preloads a special Perl library file containing the debugger.

       The program will halt right before the first run-time executable statement (but see below
       regarding compile-time statements) and ask you to enter a debugger command.  Contrary to
       popular expectations, whenever the debugger halts and shows you a line of code, it always
       displays the line it's about to execute, rather than the one it has just executed.

       Any command not recognized by the debugger is directly executed ("eval"'d) as Perl code in
       the current package.  (The debugger uses the DB package for keeping its own state

       Note that the said "eval" is bound by an implicit scope. As a result any newly introduced
       lexical variable or any modified capture buffer content is lost after the eval. The
       debugger is a nice environment to learn Perl, but if you interactively experiment using
       material which should be in the same scope, stuff it in one line.

       For any text entered at the debugger prompt, leading and trailing whitespace is first
       stripped before further processing.  If a debugger command coincides with some function in
       your own program, merely precede the function with something that doesn't look like a
       debugger command, such as a leading ";" or perhaps a "+", or by wrapping it with
       parentheses or braces.

   Calling the Debugger
       There are several ways to call the debugger:

       perl -d program_name
           On the given program identified by "program_name".

       perl -d -e 0
           Interactively supply an arbitrary "expression" using "-e".

       perl -d:ptkdb program_name
           Debug a given program via the "Devel::ptkdb" GUI.

       perl -dt threaded_program_name
           Debug a given program using threads (experimental).

   Debugger Commands
       The interactive debugger understands the following commands:

       h           Prints out a summary help message

       h [command] Prints out a help message for the given debugger command.

       h h         The special argument of "h h" produces the entire help page, which is quite

                   If the output of the "h h" command (or any command, for that matter) scrolls
                   past your screen, precede the command with a leading pipe symbol so that it's
                   run through your pager, as in

                       DB> |h h

                   You may change the pager which is used via "o pager=..." command.

       p expr      Same as "print {$DB::OUT} expr" in the current package.  In particular,
                   because this is just Perl's own "print" function, this means that nested data
                   structures and objects are not dumped, unlike with the "x" command.

                   The "DB::OUT" filehandle is opened to /dev/tty, regardless of where STDOUT may
                   be redirected to.

       x [maxdepth] expr
                   Evaluates its expression in list context and dumps out the result in a pretty-
                   printed fashion.  Nested data structures are printed out recursively, unlike
                   the real "print" function in Perl.  When dumping hashes, you'll probably
                   prefer 'x \%h' rather than 'x %h'.  See Dumpvalue if you'd like to do this

                   The output format is governed by multiple options described under
                   "Configurable Options".

                   If the "maxdepth" is included, it must be a numeral N; the value is dumped
                   only N levels deep, as if the "dumpDepth" option had been temporarily set to

       V [pkg [vars]]
                   Display all (or some) variables in package (defaulting to "main") using a data
                   pretty-printer (hashes show their keys and values so you see what's what,
                   control characters are made printable, etc.).  Make sure you don't put the
                   type specifier (like "$") there, just the symbol names, like this:

                       V DB filename line

                   Use "~pattern" and "!pattern" for positive and negative regexes.

                   This is similar to calling the "x" command on each applicable var.

       X [vars]    Same as "V currentpackage [vars]".

       y [level [vars]]
                   Display all (or some) lexical variables (mnemonic: "mY" variables) in the
                   current scope or level scopes higher.  You can limit the variables that you
                   see with vars which works exactly as it does for the "V" and "X" commands.
                   Requires the "PadWalker" module version 0.08 or higher; will warn if this
                   isn't installed.  Output is pretty-printed in the same style as for "V" and
                   the format is controlled by the same options.

       T           Produce a stack backtrace.  See below for details on its output.

       s [expr]    Single step.  Executes until the beginning of another statement, descending
                   into subroutine calls.  If an expression is supplied that includes function
                   calls, it too will be single-stepped.

       n [expr]    Next.  Executes over subroutine calls, until the beginning of the next
                   statement.  If an expression is supplied that includes function calls, those
                   functions will be executed with stops before each statement.

       r           Continue until the return from the current subroutine.  Dump the return value
                   if the "PrintRet" option is set (default).

       <CR>        Repeat last "n" or "s" command.

       c [line|sub]
                   Continue, optionally inserting a one-time-only breakpoint at the specified
                   line or subroutine.

       l           List next window of lines.

       l min+incr  List "incr+1" lines starting at "min".

       l min-max   List lines "min" through "max".  "l -" is synonymous to "-".

       l line      List a single line.

       l subname   List first window of lines from subroutine.  subname may be a variable that
                   contains a code reference.

       -           List previous window of lines.

       v [line]    View a few lines of code around the current line.

       .           Return the internal debugger pointer to the line last executed, and print out
                   that line.

       f filename  Switch to viewing a different file or "eval" statement.  If filename is not a
                   full pathname found in the values of %INC, it is considered a regex.

                   "eval"ed strings (when accessible) are considered to be filenames: "f (eval
                   7)" and "f eval 7\b" access the body of the 7th "eval"ed string (in the order
                   of execution).  The bodies of the currently executed "eval" and of "eval"ed
                   strings that define subroutines are saved and thus accessible.

       /pattern/   Search forwards for pattern (a Perl regex); final / is optional.  The search
                   is case-insensitive by default.

       ?pattern?   Search backwards for pattern; final ? is optional.  The search is case-
                   insensitive by default.

       L [abw]     List (default all) actions, breakpoints and watch expressions

       S [[!]regex]
                   List subroutine names [not] matching the regex.

       t [n]       Toggle trace mode (see also the "AutoTrace" option).  Optional argument is the
                   maximum number of levels to trace below the current one; anything deeper than
                   that will be silent.

       t [n] expr  Trace through execution of "expr".  Optional first argument is the maximum
                   number of levels to trace below the current one; anything deeper than that
                   will be silent.  See "Frame Listing Output Examples" in perldebguts for

       b           Sets breakpoint on current line

       b [line] [condition]
                   Set a breakpoint before the given line.  If a condition is specified, it's
                   evaluated each time the statement is reached: a breakpoint is taken only if
                   the condition is true.  Breakpoints may only be set on lines that begin an
                   executable statement.  Conditions don't use "if":

                       b 237 $x > 30
                       b 237 ++$count237 < 11
                       b 33 /pattern/i

                   If the line number is ".", sets a breakpoint on the current line:

                       b . $n > 100

       b [file]:[line] [condition]
                   Set a breakpoint before the given line in a (possibly different) file.  If a
                   condition is specified, it's evaluated each time the statement is reached: a
                   breakpoint is taken only if the condition is true.  Breakpoints may only be
                   set on lines that begin an executable statement.  Conditions don't use "if":

                       b lib/ $x > 30
                       b /usr/lib/perl5/site_perl/ ++$count100 < 11

       b subname [condition]
                   Set a breakpoint before the first line of the named subroutine.  subname may
                   be a variable containing a code reference (in this case condition is not

       b postpone subname [condition]
                   Set a breakpoint at first line of subroutine after it is compiled.

       b load filename
                   Set a breakpoint before the first executed line of the filename, which should
                   be a full pathname found amongst the %INC values.

       b compile subname
                   Sets a breakpoint before the first statement executed after the specified
                   subroutine is compiled.

       B line      Delete a breakpoint from the specified line.

       B *         Delete all installed breakpoints.

       disable [file]:[line]
                   Disable the breakpoint so it won't stop the execution of the program.
                   Breakpoints are enabled by default and can be re-enabled using the "enable"

       disable [line]
                   Disable the breakpoint so it won't stop the execution of the program.
                   Breakpoints are enabled by default and can be re-enabled using the "enable"

                   This is done for a breakpoint in the current file.

       enable [file]:[line]
                   Enable the breakpoint so it will stop the execution of the program.

       enable [line]
                   Enable the breakpoint so it will stop the execution of the program.

                   This is done for a breakpoint in the current file.

       a [line] command
                   Set an action to be done before the line is executed.  If line is omitted, set
                   an action on the line about to be executed.  The sequence of steps taken by
                   the debugger is

                     1. check for a breakpoint at this line
                     2. print the line if necessary (tracing)
                     3. do any actions associated with that line
                     4. prompt user if at a breakpoint or in single-step
                     5. evaluate line

                   For example, this will print out $foo every time line 53 is passed:

                       a 53 print "DB FOUND $foo\n"

       A line      Delete an action from the specified line.

       A *         Delete all installed actions.

       w expr      Add a global watch-expression. Whenever a watched global changes the debugger
                   will stop and display the old and new values.

       W expr      Delete watch-expression

       W *         Delete all watch-expressions.

       o           Display all options.

       o booloption ...
                   Set each listed Boolean option to the value 1.

       o anyoption? ...
                   Print out the value of one or more options.

       o option=value ...
                   Set the value of one or more options.  If the value has internal whitespace,
                   it should be quoted.  For example, you could set "o pager="less -MQeicsNfr""
                   to call less with those specific options.  You may use either single or double
                   quotes, but if you do, you must escape any embedded instances of same sort of
                   quote you began with, as well as any escaping any escapes that immediately
                   precede that quote but which are not meant to escape the quote itself.  In
                   other words, you follow single-quoting rules irrespective of the quote; eg: "o
                   option='this isn\'t bad'" or "o option="She said, \"Isn't it?\""".

                   For historical reasons, the "=value" is optional, but defaults to 1 only where
                   it is safe to do so--that is, mostly for Boolean options.  It is always better
                   to assign a specific value using "=".  The "option" can be abbreviated, but
                   for clarity probably should not be.  Several options can be set together.  See
                   "Configurable Options" for a list of these.

       < ?         List out all pre-prompt Perl command actions.

       < [ command ]
                   Set an action (Perl command) to happen before every debugger prompt.  A multi-
                   line command may be entered by backslashing the newlines.

       < *         Delete all pre-prompt Perl command actions.

       << command  Add an action (Perl command) to happen before every debugger prompt.  A multi-
                   line command may be entered by backwhacking the newlines.

       > ?         List out post-prompt Perl command actions.

       > command   Set an action (Perl command) to happen after the prompt when you've just given
                   a command to return to executing the script.  A multi-line command may be
                   entered by backslashing the newlines (we bet you couldn't have guessed this by

       > *         Delete all post-prompt Perl command actions.

       >> command  Adds an action (Perl command) to happen after the prompt when you've just
                   given a command to return to executing the script.  A multi-line command may
                   be entered by backslashing the newlines.

       { ?         List out pre-prompt debugger commands.

       { [ command ]
                   Set an action (debugger command) to happen before every debugger prompt.  A
                   multi-line command may be entered in the customary fashion.

                   Because this command is in some senses new, a warning is issued if you appear
                   to have accidentally entered a block instead.  If that's what you mean to do,
                   write it as with ";{ ... }" or even "do { ... }".

       { *         Delete all pre-prompt debugger commands.

       {{ command  Add an action (debugger command) to happen before every debugger prompt.  A
                   multi-line command may be entered, if you can guess how: see above.

       ! number    Redo a previous command (defaults to the previous command).

       ! -number   Redo number'th previous command.

       ! pattern   Redo last command that started with pattern.  See "o recallCommand", too.

       !! cmd      Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT) See "o
                   shellBang", also.  Note that the user's current shell (well, their $ENV{SHELL}
                   variable) will be used, which can interfere with proper interpretation of exit
                   status or signal and coredump information.

       source file Read and execute debugger commands from file.  file may itself contain
                   "source" commands.

       H -number   Display last n commands.  Only commands longer than one character are listed.
                   If number is omitted, list them all.

       q or ^D     Quit.  ("quit" doesn't work for this, unless you've made an alias) This is the
                   only supported way to exit the debugger, though typing "exit" twice might

                   Set the "inhibit_exit" option to 0 if you want to be able to step off the end
                   the script.  You may also need to set $finished to 0 if you want to step
                   through global destruction.

       R           Restart the debugger by "exec()"ing a new session.  We try to maintain your
                   history across this, but internal settings and command-line options may be

                   The following setting are currently preserved: history, breakpoints, actions,
                   debugger options, and the Perl command-line options -w, -I, and -e.

       |dbcmd      Run the debugger command, piping DB::OUT into your current pager.

       ||dbcmd     Same as "|dbcmd" but DB::OUT is temporarily "select"ed as well.

       = [alias value]
                   Define a command alias, like

                       = quit q

                   or list current aliases.

       command     Execute command as a Perl statement.  A trailing semicolon will be supplied.
                   If the Perl statement would otherwise be confused for a Perl debugger, use a
                   leading semicolon, too.

       m expr      List which methods may be called on the result of the evaluated expression.
                   The expression may evaluated to a reference to a blessed object, or to a
                   package name.

       M           Display all loaded modules and their versions.

       man [manpage]
                   Despite its name, this calls your system's default documentation viewer on the
                   given page, or on the viewer itself if manpage is omitted.  If that viewer is
                   man, the current "Config" information is used to invoke man using the proper
                   MANPATH or -M manpath option.  Failed lookups of the form "XXX" that match
                   known manpages of the form perlXXX will be retried.  This lets you type "man
                   debug" or "man op" from the debugger.

                   On systems traditionally bereft of a usable man command, the debugger invokes
                   perldoc.  Occasionally this determination is incorrect due to recalcitrant
                   vendors or rather more felicitously, to enterprising users.  If you fall into
                   either category, just manually set the $DB::doccmd variable to whatever viewer
                   to view the Perl documentation on your system.  This may be set in an rc file,
                   or through direct assignment.  We're still waiting for a working example of
                   something along the lines of:

                       $DB::doccmd = 'netscape -remote';

   Configurable Options
       The debugger has numerous options settable using the "o" command, either interactively or
       from the environment or an rc file. The file is named ./.perldb or ~/.perldb under Unix
       with /dev/tty, perldb.ini otherwise.

       "recallCommand", "ShellBang"
                   The characters used to recall a command or spawn a shell.  By default, both
                   are set to "!", which is unfortunate.

       "pager"     Program to use for output of pager-piped commands (those beginning with a "|"
                   character.)  By default, $ENV{PAGER} will be used.  Because the debugger uses
                   your current terminal characteristics for bold and underlining, if the chosen
                   pager does not pass escape sequences through unchanged, the output of some
                   debugger commands will not be readable when sent through the pager.

       "tkRunning" Run Tk while prompting (with ReadLine).

       "signalLevel", "warnLevel", "dieLevel"
                   Level of verbosity.  By default, the debugger leaves your exceptions and
                   warnings alone, because altering them can break correctly running programs.
                   It will attempt to print a message when uncaught INT, BUS, or SEGV signals
                   arrive.  (But see the mention of signals in "BUGS" below.)

                   To disable this default safe mode, set these values to something higher than
                   0.  At a level of 1, you get backtraces upon receiving any kind of warning
                   (this is often annoying) or exception (this is often valuable).
                   Unfortunately, the debugger cannot discern fatal exceptions from non-fatal
                   ones.  If "dieLevel" is even 1, then your non-fatal exceptions are also traced
                   and unceremoniously altered if they came from "eval'ed" strings or from any
                   kind of "eval" within modules you're attempting to load.  If "dieLevel" is 2,
                   the debugger doesn't care where they came from:  It usurps your exception
                   handler and prints out a trace, then modifies all exceptions with its own
                   embellishments.  This may perhaps be useful for some tracing purposes, but
                   tends to hopelessly destroy any program that takes its exception handling

       "AutoTrace" Trace mode (similar to "t" command, but can be put into "PERLDB_OPTS").

       "LineInfo"  File or pipe to print line number info to.  If it is a pipe (say,
                   "|visual_perl_db"), then a short message is used.  This is the mechanism used
                   to interact with a slave editor or visual debugger, such as the special "vi"
                   or "emacs" hooks, or the "ddd" graphical debugger.

                   If 0, allows stepping off the end of the script.

       "PrintRet"  Print return value after "r" command if set (default).

       "ornaments" Affects screen appearance of the command line (see Term::ReadLine).  There is
                   currently no way to disable these, which can render some output illegible on
                   some displays, or with some pagers.  This is considered a bug.

       "frame"     Affects the printing of messages upon entry and exit from subroutines.  If
                   "frame & 2" is false, messages are printed on entry only. (Printing on exit
                   might be useful if interspersed with other messages.)

                   If "frame & 4", arguments to functions are printed, plus context and caller
                   info.  If "frame & 8", overloaded "stringify" and "tie"d "FETCH" is enabled on
                   the printed arguments.  If "frame & 16", the return value from the subroutine
                   is printed.

                   The length at which the argument list is truncated is governed by the next

                   Length to truncate the argument list when the "frame" option's bit 4 is set.

                   Change the size of code list window (default is 10 lines).

       The following options affect what happens with "V", "X", and "x" commands:

       "arrayDepth", "hashDepth"
                   Print only first N elements ('' for all).

       "dumpDepth" Limit recursion depth to N levels when dumping structures.  Negative values
                   are interpreted as infinity.  Default: infinity.

       "compactDump", "veryCompact"
                   Change the style of array and hash output.  If "compactDump", short array may
                   be printed on one line.

       "globPrint" Whether to print contents of globs.

                   Dump arrays holding debugged files.

                   Dump symbol tables of packages.

                   Dump contents of "reused" addresses.

       "quote", "HighBit", "undefPrint"
                   Change the style of string dump.  The default value for "quote" is "auto"; one
                   can enable double-quotish or single-quotish format by setting it to """ or
                   "'", respectively.  By default, characters with their high bit set are printed

       "UsageOnly" Rudimentary per-package memory usage dump.  Calculates total size of strings
                   found in variables in the package.  This does not include lexicals in a
                   module's file scope, or lost in closures.

       "HistFile"  The path of the file from which the history (assuming a usable Term::ReadLine
                   backend) will be read on the debugger's startup, and to which it will be saved
                   on shutdown (for persistence across sessions). Similar in concept to Bash's
                   ".bash_history" file.

       "HistSize"  The count of the saved lines in the history (assuming "HistFile" above).

       After the rc file is read, the debugger reads the $ENV{PERLDB_OPTS} environment variable
       and parses this as the remainder of a "O ..."  line as one might enter at the debugger
       prompt.  You may place the initialization options "TTY", "noTTY", "ReadLine", and
       "NonStop" there.

       If your rc file contains:

         parse_options("NonStop=1 LineInfo=db.out AutoTrace");

       then your script will run without human intervention, putting trace information into the
       file db.out.  (If you interrupt it, you'd better reset "LineInfo" to /dev/tty if you
       expect to see anything.)

       "TTY"       The TTY to use for debugging I/O.

       "noTTY"     If set, the debugger goes into "NonStop" mode and will not connect to a TTY.
                   If interrupted (or if control goes to the debugger via explicit setting of
                   $DB::signal or $DB::single from the Perl script), it connects to a TTY
                   specified in the "TTY" option at startup, or to a tty found at runtime using
                   the "Term::Rendezvous" module of your choice.

                   This module should implement a method named "new" that returns an object with
                   two methods: "IN" and "OUT".  These should return filehandles to use for
                   debugging input and output correspondingly.  The "new" method should inspect
                   an argument containing the value of $ENV{PERLDB_NOTTY} at startup, or
                   "$ENV{HOME}/.perldbtty$$" otherwise.  This file is not inspected for proper
                   ownership, so security hazards are theoretically possible.

       "ReadLine"  If false, readline support in the debugger is disabled in order to debug
                   applications that themselves use ReadLine.

       "NonStop"   If set, the debugger goes into non-interactive mode until interrupted, or
                   programmatically by setting $DB::signal or $DB::single.

       Here's an example of using the $ENV{PERLDB_OPTS} variable:

           $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram

       That will run the script myprogram without human intervention, printing out the call tree
       with entry and exit points.  Note that "NonStop=1 frame=2" is equivalent to "N f=2", and
       that originally, options could be uniquely abbreviated by the first letter (modulo the
       "Dump*" options).  It is nevertheless recommended that you always spell them out in full
       for legibility and future compatibility.

       Other examples include

           $ PERLDB_OPTS="NonStop LineInfo=listing frame=2" perl -d myprogram

       which runs script non-interactively, printing info on each entry into a subroutine and
       each executed line into the file named listing.  (If you interrupt it, you would better
       reset "LineInfo" to something "interactive"!)

       Other examples include (using standard shell syntax to show environment variable

         $ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
             perl -d myprogram )

       which may be useful for debugging a program that uses "Term::ReadLine" itself.  Do not
       forget to detach your shell from the TTY in the window that corresponds to /dev/ttyXX,
       say, by issuing a command like

         $ sleep 1000000

       See "Debugger Internals" in perldebguts for details.

   Debugger Input/Output
       Prompt  The debugger prompt is something like


               or even


               where that number is the command number, and which you'd use to access with the
               built-in csh-like history mechanism.  For example, "!17" would repeat command
               number 17.  The depth of the angle brackets indicates the nesting depth of the
               debugger.  You could get more than one set of brackets, for example, if you'd
               already at a breakpoint and then printed the result of a function call that itself
               has a breakpoint, or you step into an expression via "s/n/t expression" command.

       Multiline commands
               If you want to enter a multi-line command, such as a subroutine definition with
               several statements or a format, escape the newline that would normally end the
               debugger command with a backslash.  Here's an example:

                     DB<1> for (1..4) {         \
                     cont:     print "ok\n";   \
                     cont: }

               Note that this business of escaping a newline is specific to interactive commands
               typed into the debugger.

       Stack backtrace
               Here's an example of what a stack backtrace via "T" command might look like:

                $ = main::infested called from file '' line 10
                @ = Ambulation::legs(1, 2, 3, 4) called from file 'camel_flea'
                                                                         line 7
                $ = main::pests('bactrian', 4) called from file 'camel_flea'
                                                                         line 4

               The left-hand character up there indicates the context in which the function was
               called, with "$" and "@" meaning scalar or list contexts respectively, and "."
               meaning void context (which is actually a sort of scalar context).  The display
               above says that you were in the function "main::infested" when you ran the stack
               dump, and that it was called in scalar context from line 10 of the file
     , but without any arguments at all, meaning it was called as
               &infested.  The next stack frame shows that the function "Ambulation::legs" was
               called in list context from the camel_flea file with four arguments.  The last
               stack frame shows that "main::pests" was called in scalar context, also from
               camel_flea, but from line 4.

               If you execute the "T" command from inside an active "use" statement, the
               backtrace will contain both a "require" frame and an "eval" frame.

       Line Listing Format
               This shows the sorts of output the "l" command can produce:

                  DB<<13>> l
                101:        @i{@i} = ();
                102:b       @isa{@i,$pack} = ()
                103             if(exists $i{$prevpack} || exists $isa{$pack});
                104     }
                106     next
                107==>      if(exists $isa{$pack});
                109:a   if ($extra-- > 0) {
                110:        %isa = ($pack,1);

               Breakable lines are marked with ":".  Lines with breakpoints are marked by "b" and
               those with actions by "a".  The line that's about to be executed is marked by

               Please be aware that code in debugger listings may not look the same as your
               original source code.  Line directives and external source filters can alter the
               code before Perl sees it, causing code to move from its original positions or take
               on entirely different forms.

       Frame listing
               When the "frame" option is set, the debugger would print entered (and optionally
               exited) subroutines in different styles.  See perldebguts for incredibly long
               examples of these.

   Debugging Compile-Time Statements
       If you have compile-time executable statements (such as code within BEGIN, UNITCHECK and
       CHECK blocks or "use" statements), these will not be stopped by debugger, although
       "require"s and INIT blocks will, and compile-time statements can be traced with the
       "AutoTrace" option set in "PERLDB_OPTS").  From your own Perl code, however, you can
       transfer control back to the debugger using the following statement, which is harmless if
       the debugger is not running:

           $DB::single = 1;

       If you set $DB::single to 2, it's equivalent to having just typed the "n" command, whereas
       a value of 1 means the "s" command.  The $DB::trace  variable should be set to 1 to
       simulate having typed the "t" command.

       Another way to debug compile-time code is to start the debugger, set a breakpoint on the
       load of some module:

           DB<7> b load f:/perllib/lib/
         Will stop on load of 'f:/perllib/lib/'.

       and then restart the debugger using the "R" command (if possible).  One can use "b compile
       subname" for the same purpose.

   Debugger Customization
       The debugger probably contains enough configuration hooks that you won't ever have to
       modify it yourself.  You may change the behaviour of the debugger from within the debugger
       using its "o" command, from the command line via the "PERLDB_OPTS" environment variable,
       and from customization files.

       You can do some customization by setting up a .perldb file, which contains initialization
       code.  For instance, you could make aliases like these (the last one is one people expect
       to be there):

           $DB::alias{'len'}  = 's/^len(.*)/p length($1)/';
           $DB::alias{'stop'} = 's/^stop (at|in)/b/';
           $DB::alias{'ps'}   = 's/^ps\b/p scalar /';
           $DB::alias{'quit'} = 's/^quit(\s*)/exit/';

       You can change options from .perldb by using calls like this one;

           parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");

       The code is executed in the package "DB".  Note that .perldb is processed before
       processing "PERLDB_OPTS".  If .perldb defines the subroutine "afterinit", that function is
       called after debugger initialization ends.  .perldb may be contained in the current
       directory, or in the home directory.  Because this file is sourced in by Perl and may
       contain arbitrary commands, for security reasons, it must be owned by the superuser or the
       current user, and writable by no one but its owner.

       You can mock TTY input to debugger by adding arbitrary commands to @DB::typeahead. For
       example, your .perldb file might contain:

           sub afterinit { push @DB::typeahead, "b 4", "b 6"; }

       Which would attempt to set breakpoints on lines 4 and 6 immediately after debugger
       initialization. Note that @DB::typeahead is not a supported interface and is subject to
       change in future releases.

       If you want to modify the debugger, copy from the Perl library to another name
       and hack it to your heart's content.  You'll then want to set your "PERL5DB" environment
       variable to say something like this:

           BEGIN { require "" }

       As a last resort, you could also use "PERL5DB" to customize the debugger by directly
       setting internal variables or calling debugger functions.

       Note that any variables and functions that are not documented in this document (or in
       perldebguts) are considered for internal use only, and as such are subject to change
       without notice.

   Readline Support / History in the Debugger
       As shipped, the only command-line history supplied is a simplistic one that checks for
       leading exclamation points.  However, if you install the Term::ReadKey and Term::ReadLine
       modules from CPAN (such as Term::ReadLine::Gnu, Term::ReadLine::Perl, ...) you will have
       full editing capabilities much like those GNU readline(3) provides.  Look for these in the
       modules/by-module/Term directory on CPAN.  These do not support normal vi command-line
       editing, however.

       A rudimentary command-line completion is also available, including lexical variables in
       the current scope if the "PadWalker" module is installed.

       Without Readline support you may see the symbols "^[[A", "^[[C", "^[[B", "^[[D"", "^H",
       ... when using the arrow keys and/or the backspace key.

   Editor Support for Debugging
       If you have the GNU's version of emacs installed on your system, it can interact with the
       Perl debugger to provide an integrated software development environment reminiscent of its
       interactions with C debuggers.

       Recent versions of Emacs come with a start file for making emacs act like a syntax-
       directed editor that understands (some of) Perl's syntax.  See perlfaq3.

       Users of vi should also look into vim and gvim, the mousey and windy version, for coloring
       of Perl keywords.

       Note that only perl can truly parse Perl, so all such CASE tools fall somewhat short of
       the mark, especially if you don't program your Perl as a C programmer might.

   The Perl Profiler
       If you wish to supply an alternative debugger for Perl to run, invoke your script with a
       colon and a package argument given to the -d flag.  Perl's alternative debuggers include a
       Perl profiler, Devel::NYTProf, which is available separately as a CPAN distribution.  To
       profile your Perl program in the file, just type:

           $ perl -d:NYTProf

       When the script terminates the profiler will create a database of the profile information
       that you can turn into reports using the profiler's tools. See <perlperf> for details.

Debugging Regular Expressions

       "use re 'debug'" enables you to see the gory details of how the Perl regular expression
       engine works. In order to understand this typically voluminous output, one must not only
       have some idea about how regular expression matching works in general, but also know how
       Perl's regular expressions are internally compiled into an automaton. These matters are
       explored in some detail in "Debugging Regular Expressions" in perldebguts.

Debugging Memory Usage

       Perl contains internal support for reporting its own memory usage, but this is a fairly
       advanced concept that requires some understanding of how memory allocation works.  See
       "Debugging Perl Memory Usage" in perldebguts for the details.


       You do have "use strict" and "use warnings" enabled, don't you?

       perldebtut, perldebguts,, re, DB, Devel::NYTProf, Dumpvalue, and perlrun.

       When debugging a script that uses #! and is thus normally found in $PATH, the -S option
       causes perl to search $PATH for it, so you don't have to type the path or "which

         $ perl -Sd


       You cannot get stack frame information or in any fashion debug functions that were not
       compiled by Perl, such as those from C or C++ extensions.

       If you alter your @_ arguments in a subroutine (such as with "shift" or "pop"), the stack
       backtrace will not show the original values.

       The debugger does not currently work in conjunction with the -W command-line switch,
       because it itself is not free of warnings.

       If you're in a slow syscall (like "wait"ing, "accept"ing, or "read"ing from your keyboard
       or a socket) and haven't set up your own $SIG{INT} handler, then you won't be able to
       CTRL-C your way back to the debugger, because the debugger's own $SIG{INT} handler doesn't
       understand that it needs to raise an exception to longjmp(3) out of slow syscalls.