It looks like the 5.6 behaviour was declared to be a bug in December 2000, and patched by Simon Cozens by January 2001.

See this p5p message and the thread it's in for more details.

Judging from that thread, I think you can count on the 5.8 behaviour being the "correct" one, although I think there was some later debate about what "DTRT" means for tied or magical hashes.