When perl interprets "function (LIST),LIST;" as "function (LIST);LIST;", it is implicitly treating (and converting) the comma into a statement separator.

You're missing something: precedence. The comma already is a term separator. (Expressions can be terms.) Parentheses group terms to disambiguate the intended precedence.

If you were to change the behavior of parentheses in this case, you'd find all sorts of inconsistencies with how Perl behaves in other cases.