Thanks for the update/correction ... although saying that I'm wrong is merely semantics. The URL you point to leaves me with the impression that Larry is implying that this could change in the future - whether that is within Perl5 (as the implication at the time could have been) or Perl6, is left open. Given that, I would suggest that any statement implying that this code "works" is misleading. It happens to work with a current version of perl, but is open to breakage under newer versions of perl (including PONIE), with Larry not sounding too concerned about it.
I'm somewhat curious if intervening memory usage could clobber this trick - e.g., calling IsMatrix early, then using 200MB of memory, then coming back and calling IsMatrix a second time. Or does this trick make Perl think it's static (closed-off scope with lexical references in a global sub), thus leaking memory where, without the if 0, it would not be a memory leak? (I realise that "memory leak" is debatable here.)