Perfect Head, sadly, never made it to the final version of the game. There are MANY rumors about it. But the one that I believe is true is that: Some psycho kid would put all of his class-mates on the head of the simulants, start shooting them. Get obsessed. Then go to school and blow their heads off!. So Rare took that feature out cause they thought that would happen and they would get sued (or however ya spell it).
People have been trying to access it somehow with their gameshark. No success yet. BUT I have heard that at some website. I don't remember the name BUT they say they have unlocked Perfect Head! But this is probably another rumor.
Here are a few screenshots of Perfect Head:
