In a small bowl, combine the brown sugar and cornstarch. Set aside.
In a saucepan, bring the orange juice, rum, and raisins to a boil. Simmer gently for about 10 minutes or until the raisins are properly hydrated. Add the brown sugar mixture and bring to a boil while stirring. Simmer for 1 minute and pour into a bowl. Cover with plastic wrap directly on the surface of the mixture. Let cool.
With the rack in the lowest position, preheat the oven to 200 °C (400 °F).
Line a 23-cm (9-inch) pie plate with one pastry. Spoon the filling in the pie and brush the edges of the dough with milk. Top with the second pastry. Make an incision in the centre. Remove any excess dough. Tightly seal the edges by crimping the dough with a fork or your fingers. Brush with milk.
Bake for about 40 minutes or until the crust is golden brown. Serve warm or cold.
If you prefer this pie without alcohol, replace the rum with water.