Peel and core the pears and cut into eighths; set aside.
In a medium-size saucepan, combine the brown sugar, cornstarch and cinnamon or allspice. Stir in the juice or fruit nectar and mix well. Place the pan on medium-high heat and cook the sauce, stirring, until the mixture is smooth. Add the pears and bring the sauce to a gentle boil.
Reduce the heat to medium-low and continue cooking, stirring occasionally, for 15 - 20 minutes or until the pears are just tender, almost translucent, and the syrup is thick. Stir in the rum and butter and heat a few minutes longer.