But I think it's important to point out that hash ordering has always been random, even though older Perls used to return hash keys with some consistency to the ordering, leading to code sometimes relying on this fact. But at least since v5.18.0 that has changed, see Hash overhaul. The bottom line is, one shouldn't rely on the order hash keys are returned in, in any version of Perl.

Actually it's much older than that. In order to protect perl from some form of DOS attacks, a randomization of key ordering was implemented in perls as old as 5.8.1, so that the next time you run it, you'd most likely get a different keys order for the same hash keys inserted in the same order.

The reason for that is a possible DOS attack over hash collisions (from 2003) which was fixed that same year in October.