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;".