=over =item $EFFECTIVE_USER_ID =item $EUID =item $> X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID> The effective uid of this process. For example: $< = $>; # set real to effective uid ($<,$>) = ($>,$<); # swap real and effective uids You can change both the effective uid and the real uid at the same time by using C. Changes to C<< $> >> require a check to C<$!> to detect any possible errors after an attempted change. C<< $< >> and C<< $> >> can be swapped only on machines supporting C. Mnemonic: it's the uid you went I, if you're running setuid. =back