Abstract: An apparatus for perforating the minor diameter or inner corrugation of a transversely corrugated thermoplastic drainage pipe. A plurality of perforations are simultaneously formed in the inner corrugations of the pipe by reciprocally moving heated punches radially into the pipe, removing the punches and then translating the pipe to another position for the next punching step. In an alternative embodiment, the punches are carried by a rotating ring which positions them opposite an inner corrugation and a cam device drives the punch radially into the pipe.