To weed out images that aren't exact copies you might want to use Digest::MD5 with special attention to $md5->addfile($io_handle).

It would allow you to get MD5 checksums of your images in hex that you could take and compare. If the checksums are of you could start to search for shifted pixels.

As to the image library you might want to take a look at ImageMagick and the perl module PerlMagick, which allows the use of the library from within perl.

I hope this helps in scratching your programmers itch.

/oliver/