Your test is sound, but the answer is probably that it depends what happens when 'assignment fails'. An array in boolean context (is there such a thing in perl?) will evaluate to true if it contains anything at all:

() is false
(undef) is true
(0) is true
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so if it's coming from a sub that explicitly returns 0 or undef on failure, you'll end up with a populated array. The best thing to do is use just return, since return undef is true in list context. I dimly recall that there occasions when return () is dodgy in scalar context, too, but I can't remember what they are :(

hth