It's certainly true that Perl6::Form only works with Perl 5.8 and later, so you can't use it if you're still on 5.6.
But I think your concern about source filtering is misplaced in this instance. Perl6::Form doesn't use a source filter on your code - it uses it on its own code. The problem with source filters that attempt to add syntax to Perl is that they will never parse the language completely accurately, and so will misbehave on certain code. (That's why Switch.pm is unreliable, for example.) In this case, the code that's being filtered is the source code of Perl6::Form, which obviously won't change, and has been verified to work with the Perl6::Export filter.