If you can find templates that "are not supposed to change", like the page for a book cover or something like that, maybe you can set up a special single-page document and render that to a bitmap using (yuck) Image::Magick (or maybe better direct Ghostscript). Then you can try to use Image::Compare or Image::SubImageFind to find the "not changing" parts again.
Of course, maintaining such a library of image-based tests gets really ugly. Maybe you can use wraith by the BBC to manage and compare the "screenshots" whenever a change is detected.