But, when I do !1, it is neither returning 0 nor undef.

Those aren't the only false values. For example, an empty string is also false. It's not exactly an empty string being returned, though.

It shouldn't matter to you at all what it returns as long as it's false, but the value it happens to return is zero in numeric contexts and the empty string in string contexts.

>perl -wE"say ''.!1"


>perl -wE"say 0+!1"
0
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If it was just zero, the result would be different:

>perl -wE"say ''.0"
0

>perl -wE"say 0+0"
0
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If it was just the empty string, the result would be different:

>perl -wE"say ''.''"


>perl -wE"say 0+''"
Argument "" isn't numeric in addition (+) at -e line 1.
0
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