Harnwell made the surprise retirement announcement just hours before kick-off due to serious ongoing knee problems.
The 250-game Perth Glory legend said the Gulls had been like a family to him.
‘I started playing up here when I was four or five years old,’ he said.
‘It’s been my home for a long time, and it will be for a long time to come.’
The Gulls put through two goals in the first half, with Perth unable to respond.
In ECU Joondalup news, the Jacks salvaged a last-minute 2-2 draw against Bayswater City to keep their hopes of escaping relegation alive.
The Jacks remain on the bottom of the table, but a win against easy beats the National Training Centre this Saturday could help them usurp the second-last-placed Armadale.
Third-placed Sorrento takes on fifth-placed Inglewood.