Katy Perry mocks internet sensation Rebecca Black

By GEORGINA LITTLEJOHN Mocking: Katy Perry sang an acoustic and tuneless version of Rebecca Black's Friday on stage in Melbourne last nightThere are probably a lot of teenage girls who would be thrilled to bits if Katy Perry decided to sing one of their songs during her concert.But not Rebecca Black, who will no doubt be feeling a little disappointed with the Firework star's 'tribute' to her.