Section: Perl Programmers Reference Guide (3pm)
PerlIO::mmap - Memory mapped IO
open my $fh, '<:mmap', $filename;
This layer does "read"
operations by mmap()
ing the file if possible, but falls back to the default behavior if not.
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;"