$< 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.

$> 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().

#!/usr/bin/perl

$> = 0;
system "whoami";

$> = 1000;
system "whoami";

$> = 0;
system "whoami";

# ./test.pl
root
dorainm
root

#