I'm not sure what you mean here by "two instances of the Perl parser". I'm pretty sure I know what you're getting at, but I wouldn't call it two instances of the parser. It is more like two possible syntactic interpretations/parses of the same piece of code, depending on the prototype of a previously defined sub.

Saying "two instances of the parser" produce different things (on the same given code, presumably) makes it sound simply like the parser is non-deterministic or randomized.