my ($width,        $height       ) = ...;
my ($max_width,    $max_height   ) = (800, 600);
my ($width_factor, $height_factor) = ($max_width/$width, $max_height/$height);

if ($width_factor < 1 || $height_factor < 1) {
   my $factor = ($width_factor < $height_factor
      ? $width_factor
      : $height_factor
   );

   $width  = int($width  * $factor + 0.5);
   $height = int($height * $factor + 0.5);
}