An ex-cop caught in a lie by an attempted murderer and an MP3 player was sentenced to four months in jail yesterday for perjury.

Former Detective Christopher Perino had denied questioning Erik Crespo, 17, about a Christmas Day shooting in The Bronx in 2005. But he was blindsided by a recording Crespo had made of the 70-minute interrogation. The perjury could have landed the ex-cop in jail for up to seven years — the same sentence Crespo was handed for shooting his victim in the face.

Bronx Supreme Court Judge James Kindler called Perino’s lies “an affront to the court. It was directly due to [Perino’s] perjury that the trial fell apart, and Crespo got the benefit.”

Crespo had faced 15 years in jail when Perino lied at his trial. After the perjury was exposed, the thug was able to plea-bargain his way down to seven years. Perino claimed he’d forgotten interviewing the teen. .