Scale fish by placing it in the sink under cold running water. Grasp fish firmly by the gills and scrape off scales with a fish scaler or small, dull knife. Use short strokes and work from the tail to the head. The skin is usually cooked and eaten with the fish.
To remove the head, cut the flesh on both sides with a knife. If the fish is small, slice directly through the spine. For a larger fish, place the knife between vertebrae and tap the back of the knife with a hammer.
To fillet, use a sharp, thin knife. With the perch lying on its side, insert the knife behind the gills, and cut in an arc down to just above the backbone. Continue cutting parallel to the backbone toward the tail. Bring the knife up at the tail and remove the fillet.
The secret to successful perch cookery is to not overcook. Whichever of the following cooking methods you choose, your perch will be cooked when its flesh becomes opaque yet is still moist on the inside.
Rinse fish and pat dry with a paper towel. Large whole walleye can be stuffed before baking. Place whole or filleted perch in a baking pan and cover with sauce. Bake in the oven at 400°F (200°C) until a knife slice in the thickest part reveals the flesh to be opaque but still moist.
Rinse perch fillets and pat dry with a paper towel. Place fish on a rack above a baking dish. Brush with oil or butter or dredge in flour and seasonings, if desired. Shake off any excess flour. Preheat broiler and adjust oven rack so fish is 3 to 4 inches (about 7.6 to 10cm) from the element. Broil, turning once, until fish is opaque but still moist in the center, six to ten minutes, depending on size of the fish.
Rinse perch and pat dry with a paper towel. Dredge in flour and seasonings, if desired. Shake off any excess flour. Heat frying pan until hot. Add butter or oil. Put in fillets and cook, turning once, until opaque but still moist in the center, two to ten minutes, depending upon the size of the fish.
Pour oil into a wok or deep fryer; it should be at least 1 1/2 inches (about 3.8cm) deep, and the cooker should be less than half full of oil. Heat oil to 375°F (190°C), using a thermometer to monitor temperature. Cut perch into similar-sized pieces, 1 1/4 to 1 1/2-inch (about 3.2 to 3.8 cm) thick. Dip in batter, drain, then slip pieces into hot oil. Cook until brown, two to three minutes.
Bring poaching liquid, consisting of water, broth, and herbs and spices, to a simmer. Slip in perch, then cover pan and keep liquid at a simmer for about eight minutes per inch (about 2.5cm) thickness of the fish.
Place perch on a greased perforated rack over 1 to 2 inches (2.5 to 5cm) of rapidly boiling water. Cover with a tight-fitting lid and keep water at a constant boil through cooking time, eight to ten minutes per inch (about 2.5cm) thickness of the fish.