I suspect it has something to do with the way these things do work:

($a, $b, $c)          = (1, 2, 3);
($a, $b)              = (1, 2, 3);   # 3 discarded
($a, undef, $c)       = (1, 2, 3);   # 2 discarded
($a, undef)           = (1, 2, 3);   # 2 and 3 discarded
(undef, undef, undef) = (1, 2, 3);   # 1 2 and 3 discarded
()                    = (1, 2, 3);   # functionally equivalent
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"undef" is the only real non-variable value you can use on the left-hand-side like that.