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# $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().

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