perlre explains why this is not permitted

Actually, perlre explains why (?{ code }) is not permitted which is quite a different thing.

The original code worked exactly as expected for me with the addition of a closing ')' on the if statement. (The match fails to increment $yep because the 'v' in $word doesn't match the vowel pattern).

Using variable interpolation in a regex is not a 'weird idea' at all. It can be an excellent way to improve the readability of your code.