I'm not sure how much perl6 will allow you to mess with the parser, but source-filters in perl5 suck.

As I understand it Perl 6 will give you pretty much complete access to the parser. The Perl 6 grammar will be represented with the new regexp system so you can tweak it to your hearts content.

Actually I think this might be a first for Perl. While other languages like Lisp and Pop-11 allow you to write your own syntax with macros, etc. I can't recall a language that has an explicit representation of the grammar that you can tweak directly at runtime. Nice.