Sure, it involves more lines (12 now instead of ~8 lines), but why you would deliberately want to throw a costly sub call in a loop?

Because that way my code stays maintainable. You already have two blocks of nearly identical code, and if the call to $filter comes up in different places, you duplicated code for each of these occurrences.

If my code is too slow I can still go back and profile and optimize it.

But duplicate code kills maintainability, because if you fix your code in one place you can be sure you forget to fix it in some other cases.

My example was reduced to the bare minimum that looked useful, but more often the actually duplicated code is larger.