Preheat the oven to 350°F Butter a 9-x-5-inch loaf pan an d line th bottom with baking parchment or waxed paper.
In a medium bowl, sift together the flour, baking powder, cinnamon, nutmeg, salt, and allspice.
Place the calabaza puree, sugar, butter, and eggs in a large mixing bowl and thoroughly combine with an electric mixer. With the mixer at low speed, gradually add the flour mixture. Fold in the raisins. Spoon the batter into the prepared loaf pan.
Bake on the middle rack of the oven until the loaf has risen and a toothpick inserted in the center comes out clean, 5o to 60 minutes. Let the loaf cool in the pan on a wire rack for 10 minutes, then turn it out onto the rack, peel off the paper, and let it cool completely before serving.