memo

perldoc perlvar

       $REAL_USER_ID
       $UID
       $<      The real uid of this process.  (Mnemonic: it's the uid you came from, 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.

       $EFFECTIVE_USER_ID
       $EUID
       $>      The effective uid of this process.  Example:

                   $< = $>;            # set real to effective uid
                   ($<,$>) = ($>,$<);  # swap real and effective uid

               You can change both the effective uid and the real uid at the same time by using
               POSIX::setuid().  Changes to $> require a check to $!  to detect any possible errors after an
               attempted change.

               (Mnemonic: it's the uid you went to, if you're running setuid.)  $< and $> can be swapped only
               on machines supporting setreuid().

→よくわかりません（＞＜）とか書きたくなりますね。