This is the section right out of the Perl documentation for $$. You can find it in perlvar.

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

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