=over =item $PROCESS_ID =item $PID =item $$ X<$$> X<$PID> X<$PROCESS_ID> The process number of the Perl running this script. Though you I set this variable, doing so is generally discouraged, although it can be invaluable for some testing purposes. It will be reset automatically across C 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POSIX semantics on Linux systems using LinuxThreads, a partial implementation of POSIX Threads that has since been superseded by the Native POSIX Thread Library (NPTL).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 C like this made embedding perl unnecessarily complex (since you'd have to manually update the value of $$), so now C<$$> and C 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 C returns a false value. NTPL threads preserve the POSIX semantics. Mnemonic: same as shells. =back