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 the Courier asks Dog to show them where to hit them.
Alternatively follow a dialog path that requires the Cannibal perk, and he will give the perk soon after he is met.
Ghost people are much more likely to die outright, even without being dismembered or disintegrated. Once this perk is in effect, when ghost people are reduced to zero health, one of their limbs will explode automatically at random.