The phrase is a play off "pervert" and "to wank." Literally, means someone who masturbates with perverted qualities. It is often used in a playful sense to label someone who is not shy about hiding his sexual appetite.
Can also be used negatively to label someone as a chronic masturbater or someone with immoral sexual desires; however, it is more often used in a playful sense.
Look at that old dude flirting with all of those young girls, what a pervy wanker.