There is a project called "GiST" which is about indexing various types of data in a Database.
One part of this Project is a system called "Blobworld" which allows the user to search for similar images by selecting and rating regions of an existing Image. I don't know how "Perlish" this one is, but it could be worth a view.
You can find the Project here.
And here you can test Blobworld.

Hope this helps you
Braindead One