# select the smallest $M elements;
while ($new_element = $dbh->fetchrow) {
  if ($heap->size < $M) {               # build a heap of size M
    $heap->insert($new_element);
  } elsif ($element < $heap->largest) { # maintain a heap containing
                                        # the smallest M elements 
                                        # seen so far
    $heap->remove_largest;
    $heap->insert($element);
  }
}