Have caught loads of bream in mist and fog (when targetting tench!!) but I don't recall perch being particularly turned on, or off, by fog. They are supposed to go off the feed during high levels of sunlight so perhaps the fog would extend their feeding patterns.
Have to echo Peter J's thoughts on fishing in the fog. The peace and quiet and the sound of the river is truly therapeutic. Had a weekend away from fishing to go bunny blatting on the South Downs yesterday. Couldn't see the damn things in the fog. Should've gone fishing.