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Why is the second case unexpected?

"0 but true" is not "True".

It means "treat me in numeric context as 0, but in boolean context as true".

So when you do "0 but true" + 0 you are in numeric context, so you effectively do 0+0 which is 0 and this in turn (for the ? operator) is false.

I find that quite logical (which probably shows that using Perl for too long causes brain-damage).

And by the way: You are not doing numeric addition on a string, you are evaluating "0 but true" in numeric context (to 0) - therefore no warning.

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