$PROCESS_ID
       $PID
       $$      The process number of the Perl running this script.  You should
               consider this variable read-only, although it will be altered
               across fork() calls.  (Mnemonic: same as shells.)

               Note for Linux users: on Linux, the C functions "getpid()" and
               "getppid()" return different values from different threads. In
               order to be portable, this behavior is not reflected by $$,
               whose value remains consistent across threads. If you want to
               call the underlying "getpid()", you may use the CPAN module
               "Linux::Pid".