Complex sentence: Ronnie is the girl friend of an older gangster, Pernell, who is serving a life term in prison--but when Caine visits Pernell in prison Pernell encourages Caine to leave with Ronnie to Atlanta.
Split into simpler sentences: Ronnie is the girl friend of an older gangster, Pernell, who is serving a life term in prison--but later agrees to go to Atlanta with Ronnie. During a prison visit, Pernell encourages Caine to leave with Ronnie to Atlanta.