returns the negation of the previous hash value (undef is false, so it returns true)

Nit: post-increment returns 0 if the value was undef, not undef. This is occasionally criticised, defended, pointed out as inconsistent with post-decrement (which does return undef), and then dropped. A comment in pp.c refers one to http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/2003-03/msg00536.html for further info (where you can read the criticism, defense, etc.).