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PerlIO::mmap - Memory mapped IO
open my $fh, '<:mmap', $filename;
This layer does "read" and "write" operations by mmap()ing the file if possible, but falls back to the default behavior if not.
"PerlIO::mmap" only exists to use XSLoader to load C code that provides support for using memory mapped IO. One does not need to explicitly "use PerlIO::mmap;".