The Mmap module lets you use mmap to map in a file as a Perl variable
rather than reading the file into dynamically allocated memory.

The advantage of this is that several processes may share one copy of
the file or string, saving memory, and concurrently making changes to
portions of the file or string.  When not used with a file, it is an
alternative to SysV shared memory that places no arbitrary size limits
on the shared memory area, and efficiently handles sparse memory usage.

WWW: http://search.cpan.org/dist/Sys-Mmap/