# two image objects
my $image1 = new <some image object>;
my $image2 = new <some image object>;

# ensure images are same size
if ($image1->size != $image2->size) {
    die "Images different sizes\n";
    }

# locals
my $position = 0;
my $cumulative_distance = 0;
my %bad_pixels;
my $threshold = 100;

# loop over all pixels
while ($position < $image1->size) {

    # get current pixels
    my $pixel1 = $image1->getpixel($position);
    my $pixel2 = $image2->getpixel($position);

    my $distance = 0;

    # calculate distance for each colour
    $distance += ($pixel1->red    - $pixel2->red   )^2;
    $distance += ($pixel1->yellow - $pixel2->yellow)^2;
    $distance += ($pixel1->blue   - $pixel2->blue  )^2;

    $distance = $distance^0.5;

    # if distance very large, add to bad pixel-list
    if ($distance > $threshold) {
         $bad_pixels{$position} = $distance;
         }

    $cumulative_distance += $distance;
    $position++;

    }

my $average_distance = $cumulative_distance / $position;
my $bad_pixels = scalar(keys(%bad_pixels));

print "Total Distance: $cumulative_distance\n",
      "Avg   Distance: $average_distance\n",
      "Deviant Pixels: $bad_pixels\n";