Risdon netted the winner in the 87th minute after Glory fell behind in the first half, only to take the lead midway through the second period thanks to goals from Sidnei Sciola and Rostyn Griffiths.
Wanderers striker Kerem Bulut made it 2-2 when he capitalised on a defensive mistake from Chris Harold in the 72nd minute before Risdon stepped up with a superb strike from 23 yards.
Victory for third-placed Perth saw them haul themselves level with Melbourne Victory.
In the day's other game, Melbourne City edged to a tight 1-0 victory over a tired Brisbane Roar to cement their position in the play-off places.
Sixth-placed City are now eight points clear of the reigning champions, who have a game in hand over their direct rivals, but with only five matches remaining to try and catch John van 't Schip's men.
The home side made a fast start and took a deserved early lead through Jonatan Germano, before the title-holders, weakened by injuries and international absentees, slowly took hold of the ball, if not the contest.
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