my $uid = getpwnam("nobody");
$< = $uid;

##</code><code>##

$< = $uid;
$> = $uid;

$> = 0;
$< = 0;

##</code><code>##

$ sudo strace ./switchback 2>&1 | grep '^set'
setreuid32(99, -1)                      = 0
setresuid32(-1, 99, -1)                 = 0
setresuid32(-1, 0, -1)                  = 0
setreuid32(0, -1)                       = 0