Talk to Dog after he has killed and devoured one of the ghost people. Follow the Speech path starting with "I watched you devour one of the ghost people". The perk will be awarded once you ask Dog to show you where he hits them.
Alternatively follow a dialog path that requires the Cannibal perk, and he will give you the perk as soon as you meet him.
Ghost people are much more likely to die outright, even without being dismembered or disintegrated. Once this perk is in effect, when you reduce ghost people to zero health, one of their limbs will explode automatically at random.