Theres another distinction besides the ones already mentioned. The first one screams "I'm setting up %hash now for the first time!" the second one depending on where the my %hash; part is located may also be a merge operation. In other words one to me looks like an initialization and the other a merge or aggregation. For instance we may have multiple reports being returned and want to merge them together and for this purpose the first is unsuitable.

OTOH, if it is an initialization then I personally prefer the first style.

:-)