Before I flew with them, I searched but I couldn't find this info anywhere either.
Having now flown Qatar from Perth, I can tell you its the Malaysian lounge. This would have to be the worst lounge at Perth airport. Its very small, dominated by a blaring TV, offers a limited range of drinks and cold snacks, has two 90s-style PCs, and the worst thing is it has no toilet facilities (you have to leave the lounge to use the public toilets outside). Compared to the Emirates and Qantas lounges in Perth, this would only be rated 1 star.
Not surprising they have not stated this in their publicity anywhere. The worst lounge in the worst capital city airport in Australia is not the best introduction to an airline that claims to be five star.