Preheat oven to 350°F with rack in middle. Soak bread crumbs in milk in a large bowl. Meanwhile, cook onion, garlic, celery, and carrot in butter in a large heavy skillet over medium heat, stirring occasionally, 5 minutes. Cover skillet and reduce heat to low, then cook until carrot is tender, about 5 minutes. Remove from heat and stir in Worcestershire sauce, vinegar, allspice, 2 teaspoons salt, and 1 1/2 teaspoons pepper. Add to bread-crumb mixture. Finely chop bacon and prunes in a food processor, then add to onion mixture along with beef, pork, eggs, and parsley and mix together with your hands. Pack mixture into a 9- by 5-inch oval loaf in a 13- by 9-inch shallow baking dish or pan. Bake until an instant-read thermometer inserted into center of meatloaf registers 155°F, 1 to 1 1/4 hours. Let stand 10 minutes before serving.