The Mmap module uses the POSIX mmap call to map in a file as a
Perl variable. Memory access by mmap may be shared between threads or
forked processes, and may be a disc file that has been mapped into memory.
Sys::Mmap depends on your operating system supporting UNIX or
POSIX.1b mmap, of course.

*Note* that PerlIO now defines a ':mmap' tag and presents
mmap'd files as regular files, if that is your cup of joe.

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. Unlike SysV
shared memory, there are no arbitrary size limits on the shared memory
area, and sparse memory usage is handled optimally on most modern UNIX
implementations.