That's a fun one -

Yes - I was tempted to conclude that it must have been an insidious DOS attack launched by the sadistically evil and ever resourceful Anonymous Monk ;-)

I eventually found the answer to the question I was going to ask, with the aid of the perlapi documentation (last resort).
A pretty dumb question, anyway: "When is the condition (SvTYPE(sv) == SVt_IV && !SvIOK(sv)) ever true ?"
Naively, I was thinking that if the SV is of type SVt_IV, then its IOK flag will inevitably be set.
(Not so, of course.)

Cheers,
Rob