Puree tomatoes in a blender until smooth. If dry, add a tablespoon or two of water. Pass through a medium strainer into a mixing bowl. Stir in Pernod, salt, and pepper. Reserve.
Season shrimps with salt and pepper. Melt 4 tablespoons butter in a large skillet over high heat. Saute shrimps until pink-orange, about 1 minute per side. With a slotted spoon, transfer shrimp to a platter, leaving butter in pan. Add reserved tomato sauce and julienned snow peas. Bring to a boil and cook until reduced by about one-third. Adjust seasonings to taste.
Break remaining butter in small pieces and stir into sauce along with shrimp. Once smooth, remove from heat, and ladle over bowls of warm fettuccine. Garnish with chervil and serve immediately.