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