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