Like someone said above, image comparision is not an easy subject. You're best bet is to write it in C, and call it from perl. To pre-process the images (before you compare), you should look into color normalization. It's a simple formula actually(the image is a NxM matrix or 2d array).

foreach pixel {
     (pixel - min)*255/max
}
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If you've got RGB you'll have to do that for each of the color values. After you do that, you'll probobly wanna do edge detection using a laplachian filter. After this it gets really tricky, but learning those two things will get you pointed in the right direction.