Almost any fly will take a perch or crappie, they are not picky if you get it close to them.
Small leach patterns, like Woolly Buggers tied on size 10 hooks, work well. So do small minnow patters, like the Mutaka, again tied size 10 or a little bigger. Damsel Fly nymphs are also top notch.
I like the water boatmen, or back swimmer, patterns for early spring use. They work because they available before a lot of other bugs come on the water.
Still, any still water trout fly will catch crappie or perch if you can get it down to them.
Too much water, so many fish, too little time.