Returns the process id of the parent process.
Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work around non-POSIX thread semantics the minority of Linux systems (and Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation has since been removed. See the documentation for $$ for details.
Portability issues: getppid in perlport.
