Chop or snip the raisins (if they are large) and prunes. Place in a large container. Add the currants, lime or lemon peel, and allspice.
Pour in the rum and Port, mix well with a large spoon, then close the container with the lid. Place the container in a cool, dark place and allow the fruits to macerate and develop flavor for at least two weeks, or several weeks, occasionally stirring and turning over the fruit in the liquid, adding a tablespoon or two more of rum or Port as desired.
Note: Brunt sugar syrup (also called browning) is made from sugar that is caramelized until it liquefies and turns almost black, with a mahogany cast. This burnt sugar is not one bit sweet. It's as bitter as black coffee, and is used for coloring sauces, cakes and gravies. It also adds a nice edge to barbecue sauce. The sugar is cooked until it reaches at least 350 degrees, but for deep coloring, it must reach 360 to 375 degrees. Since at this temperature the sugar will smoke, leave the kitchen window open. And also bear in mind that the sugar is piping hot, and children should be kept out of the kitchen while you burn the sugar! Have available a candy thermometer, wooden spoon, a pastry brush, a cup of water, and heavy mitts or pot holder. Caramel Color in a powder form, made from caramelized corn syrup, can be substituted and is available at www.KingArthurFlour.com (1-800-827-6836). Stir in 1 to 2 tablespoons of the powder into cake batter for a tan color, adding a little more for a deeper color, if desired.