The Galaxy (9-2-9) extended their unbeaten streak to 11 games in front of a sellout crowd of 27,000. They lead the MLS with 36 points and haven't lost since May 1.
Perk was called into action with starter Donovan Ricketts out because of a broken left forearm and Josh Saunders serving a suspension. The second-year goalkeeper blocked Fredy Montero's penalty shot in the 18th minute, diving to his right for his first career save.
The Sounders (8-4-8) had their three-game winning streak halted, but they matched a club record with their seventh straight game without a loss.