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$EFFECTIVE_USER_ID
$EUID
$>

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 POSIX::setuid(). Changes to $> require a check to $! to detect any possible errors after an attempted change.

$< and $> can be swapped only on machines supporting setreuid().

Mnemonic: it's the uid you went to, if you're running setuid.