Source filters are bad because you get a string and parse it yourself and then get it wrong for edge cases. What if Perl already parsed a piece of source for you and handed you a data structure instead? That fixes the greatest shortcoming.

Solution 1: B::Deparse <http://yapceurope.lv/ye2011/talk/3597>, <http://act.yapc.eu/gpw2014/talk/5266> Solution 2: B::CallChecker / <http://perldoc.perl.org/perlapi.html#cv_set_call_checker>

In other words, you do have access to the parser. It's not as neat as in Lisp because of the lack of uniform syntax, but still allows for powerful program transformations: <http://p3rl.org/Kavorka::Manual::Signatures>