The Wolf Book, chapter 7, has a whole series of examples using PDL to represent and modify an image.

I have used PDL::FFT for signal processing/generation chores. To use PDL, just feed it an array reference in the constructor. An example of mine is at Re: Manipulating Audio Data in Perl.

Modifying an image file would involve rewriting the image file's headers as well as the data munging itself.

After Compline,
Zaxo