or download this
$ 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)