Jez Butterworth’s “Mojo’’ is a raw, potent and adrenaline-fueled slice of low life that immerses audiences in the dark underside of the oft-romanticized early days of rock ’n’ roll. Now at Charlestown Working Theater in a well-acted production by Theatre on Fire, under the direction of Darren Evans, “Mojo’’ delivers the visceral impact of a sock in the jaw. Like Butterworth’s “Jerusalem,’’ it is alternately funny and chilling, loaded with mouthfuls of meaty and distinctive dialogue, and marked by abrupt shifts in the balance of power, along with equally sudden eruptions of violence.