I would never presume to think I have more local knowledge of Perth than somebody actually from there, but when we had that trip on the 340 (both the 300 and the 500) I flew it upwards of 3 dozen times. Our alternate was always Learmonth or Adelaide occasionally, with Pearce in my back pocket in case of a dire emergency with no other options.
I never saw fog there. I saw howling winds and driving rain a few times but we always got in, safely and within the limitations of the aircraft. Once I took about a half hour delay short of the runway waiting for the crosswind to drop within the -300's limits....even Garuda went around that night so you know it was bad!! Forest fires obscured one end of the runway one time and we used the other end....but never fog. And to be quite honest most of the time it was clear and a million.