In a small bowl pour enough boiling water over chile peppers to cover; let stand about 20 minutes or until chile peppers soften. Drain and discard liquid.
Meanwhile, trim fat from roast. With the point of a sharp knife, pierce the roast all over, making 1/2-inch-deep slits. Place roast in a small roasting pan.
For wet rub, in a food processor or blender combine drained chile peppers, onion, sherry, vinegar, garlic, salt, paprika, marjoram, oregano, black pepper, and allspice. Cover and process or blend until nearly smooth.
Rub meat generously with the wet rub, making sure to rub it into the slits. Cover roast and chill for 4 to 24 hours.
Preheat oven to 325 degrees F. Uncover roast and spoon any of the wet rub in the bottom of the pan over the roast. Cover roasting pan with foil. Roast for 1-1/2 hours. Remove foil. Roast about 1 hour more or until roast is tender. Let stand for 10 minutes before slicing. Garnish with fresh oregano, if desired.