Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.
Bring milk to a boil in a saucepan, stirring constantly. Pour in pudding mix and remove saucepan from heat. Stir in grated carrots, 1/2 cup raisins, cinnamon, and allspice. Cool mixture until lukewarm, 5 to 10 minutes. Stir in carrot cake mix to form a thick batter.
Pour batter into the prepared pan. Scatter 2 tablespoons raisins and half of the pecans on top.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 35 minutes. Cool to room temperature, about 30 minutes.
Combine confectioners' sugar, cream cheese, butter, and vanilla extract in a large bowl; beat until frosting smooth. Spread frosting over cooled cake. Top with remaining raisins and pecans.