Sadly thats only half of the truth. Many builtins can't be modelled with prototypes and rely on magicly hidden parsing rules.

That's why I wrote syntactically resemble

And yes, this is indeed a problem. Maybe not for own function (which rarely need such things), but when you try to override built-ins. For example autodie does that, but can't handle all the variants that the core functions do.

It's one of the things we try to make better in Perl 6: no built-in should rely on magic that's not available to the user, so that everything can be overridden and mimicked. So far I think we did a rather good job at that, though much of that isn't implemented yet.

As far as we can assure that candidate lists known at compile time, it would also be possible to catch some form of errors then (like arity mismatch).