The process number of the Perl running this script. Though you can set this variable, doing so is generally discouraged, although it can be invaluable for some testing purposes. It will be reset automatically across fork() calls.
Note for Linux and Debian GNU/kFreeBSD users: Before Perl v5.16.0 perl would emulate POSIX semantics on Linux systems using LinuxThreads, a partial implementation of POSIX Threads that has since been superseded by the Native POSIX Thread Library (NPTL).
LinuxThreads is now obsolete on Linux, and caching getpid() like this made embedding perl unnecessarily complex (since you'd have to manually update the value of $$), so now $$ and getppid() will always return the same values as the underlying C library.
Debian GNU/kFreeBSD systems also used LinuxThreads up until and including the 6.0 release, but after that moved to FreeBSD thread semantics, which are POSIX-like.
To see if your system is affected by this discrepancy check if getconf GNU_LIBPTHREAD_VERSION | grep -q NPTL returns a false value. NTPL threads preserve the POSIX semantics.
Mnemonic: same as shells.