Article continues below advertisementKaty Perry is letting her ex hear her "roar."The pop star, 40, couldn't hold back her frustration after a fan proposed she sing a John Mayer song on her Lifetimes Tour.Article continues below advertisementSource: John Mayer/YouTubeJohn Mayer and Katy Perry previously released a song together.A concertgoer brought a sign that gave Perry three options of songs to cover, though each choice named her 2013 duet, "Who You Love," with her former boyfriend."I'm not singing that song! Get away from me, Satan! How dare you!" she exclaimed.