I haven't completed the entire map yet, but I figured I would provide some information for anyone who is struggling.





The skulls are littered in quite literally every room among the map. Each time you collect around 10-20ish skulls, you get a free prize. From my play-through, I found that at 10 skulls, I got a free perk; at 20, I received another perk. At 30 or 40, I got a free pack-a-punched gun. I got to around 52 skulls, and at 50 you receive a free ray gun, but basically 1000x better.