Yes, there actually is a lounge at PH (having used it a couple of times), although it is hard to find and is not actually publicly labelled as such.
It is through a door immediately to the left of the Qantas check in in the terminal, but the door is locked, so the check-in attendant has to buzz you in.
It gets quite full about 40 minutes before the flight back to Perth (at least in the evenings), so it's worthwhile getting there a bit earlier. Nothing snazzy, but a couple of well stocked fridges and snacks - and a whole lot better than sitting anywhere else in the terminal !!