A more precise definition is version dependent. which suggests that there may be some versions out there where the false value returned is not zero

That sentence was introduced along with the change that scalar(%hash) returns the same thing as 0+keys(%hash) instead of the bucket counts, so I suspect that's what it is referring to.

But anyway, when the Perl documentation says "a false value" or "false", I don't make any assumptions of which of the possible false values it'll return :-)