Only perl can parse Perl as the language is exceptionally dynamic. It's more than possible, especially with source filters, to redefine the language's semantics on the fly.

For an example consider that any program which claims to 'parse Perl' should be able to handle the results of the much loved Acme::Buffy module (which turns your entire code into the word 'Buffy' over and over, without affecting it's functionality) and you'll begin to see the issue.

Now go one step further, ask the user whether they want to deBuffy source files and suddenly you have two different interpretations of the same code. Both interpretations are valid depending upon context, resulting in us not being able to parse both correctly as they're dependant upon state.