=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 fork() calls.  (Mnemonic: same as shells.)

Note for Linux users: on Linux, the C functions C<getpid()> and
C<getppid()> 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<getpid()>,
you may use the CPAN module C<Linux::Pid>.

=back