The process number of the Perl running this script. You should consider this variable read-only, although it will be altered across
Note for Linux users: on Linux, the C functions
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
Mnemonic: same as shells.