perldoc Imager::Filters has a section on image difference:

You can create a new image that is the difference between
       2 other images.

         my $diff = $img->difference(other=>$other_img);

       For each pixel in $img that is different to the pixel  in $othe
+r_img, the pixel from $other_img is given, otherwise
       the pixel is transparent black.
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