=over =item $REAL_USER_ID =item $UID =item $< X<< $< >> X<$UID> X<$REAL_USER_ID> The real uid of this process. (Mnemonic: it's the uid you came I, if you're running setuid.) You can change both the real uid and the effective uid at the same time by using POSIX::setuid(). Since changes to $< require a system call, check $! after a change attempt to detect any possible errors. =back