That's essentially what the 2d hash does.

$buckets->{latitude}{longitude}. You're only checking things with a similar latitude, and among those, only the things with a similar longitude. The upside is you don't need to loop. Instead of doing a latitude compare against everything, you immediately O(1) have the short list of things of the same latitude. Then instead of doing a compare against the longitudes of everything remaining, you immediately have the short list of things that match both.