I mean, it's technically parseable, since otherwise perl (the program) wouldn't be able to function. However, to parse it requires - as that article demonstrates - a preexisting knowledge of what certain words mean, and those meanings are dependent on context. That's easy for perl, since it already has to know what "whatever" means in order to compile/execute it, but is - if not impossible - than prohibitively difficult for something like a syntax highlighter or other tool meant to parse Perl without executing it per se.

Perl is, in this regard, very similar to natural language, which should be entirely unsurprising given that Larry Wall comes from a linguistics background and explicitly designed Perl to borrow various characteristics from natural languages.

From what I understand, though, Perl6's grammar has significantly fewer of these sorts of parsing complexities ("fixing" them was one of the design goals, IIRC).