Try it the other way round (first effective, then real): 

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

[download]
$ sudo strace ./switchback.pl 2>&1 | grep '^setr'
setresuid(-1, 65534, -1)                = 0
setreuid(65534, 4294967295)             = 0
setresuid(-1, 0, -1)                    = -1 EPERM (Operation not perm
+itted)
setreuid(0, 4294967295)                 = -1 EPERM (Operation not perm
+itted)

[download]

In reply to Re: $< and setuid: quite different animals? by almut
in thread $< and setuid: quite different animals? by saintmike

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