Anyone know what aspects of perl should make it impossible to define it in BNF?

Perl's grammar is overall just too general for BNF. I don't think there's an exhaustive list of all the linguistic features in Perl that make it too general for BNF, but I can tell you that it's not just a couple of things, syntactically. If there's one overall general thing to blame, it's context. To parse Perl, you have to keep track of what context any given bit of code that you're parsing is in. A given syntactic construct can have a different semantic meaning (and possibly different syntactic implications for the surrounding code) if it is encountered in a different context. BNF doesn't deal well with that.

FWIW, many overly simplistic syntax highlighting engines can't handle Perl, either, for roughly the same reason: Perl is very expressive, and therefore very complicated to parse. Not as complicated (or as expressive) as, say, English, but complicated nonetheless.