What kind of performance impact does it have? Obviously it depends on the specific program, but let's say the worst case scenario, something like a recursive implementation of the factorial function.

> What kind of performance impact does it have?

Minor. Faster unpacking of @_, but it's not a huge win until you have a lot of arguments. The conventional Perl 5 interpreter has no JIT to leverage the benefits of stronger types, inline functions, unroll loops, etc. A factorial function has few arguments, so the unpack gain will be small to nothing.