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But, having dug in to Perl Guts more than I should have, I think that disallowing tied (and probably other magic) variables on the RHS, a built-in smart match would be able to reliably determine if the RHS is a number.
The problem is that Perl doesn't expose the concept "this scalar is a number" to the user (by design). Thus making a decision based on whether a scalar is a number is nearly always wrong.
A piece of code that already makes such a decision is the code that decides whether to warn on a numeric operation:
So, what do I keep complaining about? A real problem are dual vars. Those aren't just a rare corner case that should be avoided, but for example the result from boolean operators:
So it's a number, but it's also an empty string. Should smart-matching against that be numeric or string comparison? My intuitive reaction is "string comparison", because $false doesn't round-trip when converted to a number and then to a string.
But you can also construct valid cases where round-tripping to a number is the wrong criterion; an example is if a user-supplied string is never used as a number, but happens to look like a number. You certainly don't want those values to try to coerce your own strings to numbers (and warn).
So, however you decide whether a scalar is number or a string for the purpose of comparision, I can point out a case where your decision is a big WTF. Which is precisely the reason that we have separate == and eq operators.
Whatever will be done about that, the string/number duality will remain a weakness of any Perl 5 smartmatch proposal.