=over =item $PROCESS_ID =item $PID =item $$ X<$$> X<$PID> X<$PROCESS_ID> The process number of the Perl running this script. You should consider this variable read-only, although it will be altered across C calls. Note for Linux users: on Linux, the C functions C and C return different values from different threads. In order to be portable, this behavior is not reflected by C<$$>, whose value remains consistent across threads. If you want to call the underlying C, you may use the CPAN module C. Mnemonic: same as shells. =back