%grid_size_thresholds = (
    1 => 3, # a 1x1 square grid is hot if it has 3 probs
    2 => 10,# a 2x2 square grid is hot if it has 10 probs
    3 => 30,# ...
    ...
)
@hotspots = ();

foreach $size (sort keys %grid_size_thresholds) {
    for ($x = 0; $x + $size < $GRID_WIDTH; $x++) {
       for ($y = 0; $y + $size < $GRID_HEIGHT; $y++) {
          $region = new Region($x, $x + $size,
                               $y, $y + $size);
          $prob_count = 
               get_num_probs_in_region($region);
          if ($grid_size_thresholds{$size} <= $prob_count) {
             push @hotspots, $region;
          }
       }
    }
}