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()>,
-consider using C<Inline::C> or another way to call a C library function.
+you may use the CPAN module C<Linux::Pid>.
=item $REAL_USER_ID