=over

=item $REAL_USER_ID

=item $UID

=item $<
X<< $< >> X<$UID> X<$REAL_USER_ID>

The real uid of this process.  You can change both the real uid and the
effective uid at the same time by using C<POSIX::setuid()>.  Since
changes to C<< $< >> require a system call, check C<$!> after a change
attempt to detect any possible errors.

Mnemonic: it's the uid you came I<from>, if you're running setuid.

=back