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 functionsgetpid() andgetppid() 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 underlyinggetpid(), you may use the CPAN moduleLinux::Pid.
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