=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. 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, if you're running setuid. =back