Ah, so that's why I was puzzled - the hint on each was there the first I wanted to know about deleting keys while iterating. But you said three of the following will not necessarily work:

1. map { delete $hash{$_} } keys %hash;
2. while (my ($k,$v) = each %hash) { delete $hash{$k} };
3. for (keys %hash) { delete $hash{$_} };
4. delete $hash{$_} for keys %hash;

With the each solution being valid that's one down, two to go, and now I'm more confused than before.. of the remaining three solutions, the for loops are identical except for syntactical details. So they must either both be valid or both invalid; then the map version must be valid and the for ones not. But I can't see any reason to conclude that any of them (all four) are not guaranteed to work.