while(readdir(DIRHANDLE))
  {
  if(s/^clocks//)
    {
    $thisline{category} = 'clocks';
    }
  if(s/^birds//)
    {
    $thisline{category} = 'birds';
    }
  ...
  if(! defined($thisline{category}))
    {
    die "No category defined!\n";
    }
  if(s/^vert//)
    {
    $thisline{category} = 'vert';
    }
  ... # And so on, for 2nd level subcategories.
  }