Carl was a stuntman.
History
Scooby-Doo, Where Are You!
Season one
He dressed upas the Ape Man because he was mad about not getting the main part in a movie, so tried to sabotage the set. He was undone by Shaggy Rogers, who with a Polaroid camera, managed to take a picture showing Carl in a mirror without his mask.
Carl also briefly impersonated Scooby-Doo by wearing a mask of him when confronting the real Scooby at a phony mirror.