I don't recall the arguments on each side

There are probably many more arguments for both sides, but I remember reading somewhere that one of the reasons postderef won is that some of the function concerned by the autoderef feature work on either hashes or arrays. So with each $hash{THING};, if %hash is empty, the target of each could either be autovivified into a hash or an array, or this could throw an error. But there's also keys $obj; where $obj can overload the dereferencing operators @{} and %{} and perl would have to choose which one to call (or again, throw an error). each $hash{RUD}->@* or each $obj->%* leaves no such ambiguity.