The answer for this, like many other questions, is "it depends". Humans don't do it like this at all -- we try to recognize the subject first, then find it's edges. (I suspect.) It's difficult to define what the subject /is/ in many photographs. The best way to find a method is probably to load up your stuff into an image editor, try it's various edge-detecting methods, and decide which one does the best job. Then either find a method to simply use the graphics program in question (Hint: Gimp), or do some research and find out what method the graphics program used, and how to program it in perl (Hint: PDL).

The problem is really /much/ harder then you think, though. Look at The Kiss (Safe For Work, unless you have a very strict work), for example. It's even already monochrome. The hardest border, though, is between the girls' hair and the bed. Does that mean their hair is the subject of the photograph? No, in order to find the subject, you need to follow the borders of both girls -- even where there isn't much contrast, such as between the white shirts (esp on the girl on the right) and panties and the white bed.

Update: better link.