Roll out one ball of pie dough on a lightly floured surface to 12-inch diameter. Line the bottom of a 9-inch pie pan with the dough. Chill in refrigerator while you prepare the fruit.

Stir sugar and quick cooking tapioca pearls together in a large bowl. Add the rhubarb, strawberries, and other berries. Add the lemon juice, vanilla, and mace or nutmeg. Stir gently to coat berries with the sugar. Let sit for 15 minutes, stirring once. Preheat oven to 375°F.

Roll out second ball of pie dough.

Scoop fruit mixture into dough-lined pie dish.

If you would like a lattice top, weave strips of pie dough over the top of the fruit-filled pie dish as described in How to Make a Lattice Top for a Pie Crust. (If you would like a solid top for your pie, place the second rolled-out pie dough crust on top of the pie.) Press ends of strips into the rim of the bottom crust. Trim the edges to 1/2-inch. Fold the edges back over themselves and crimp to seal. If you are using a solid top crust, score the top to create air vents.

To protect the crust edges from burning, cover the edges of the crust with aluminum foil or an aluminum pie protector. Bake in the middle rack for 25 minutes. Remove the foil. Return to oven and bake for an additional 20-30 minutes, until the top is golden and the juices are bubbling. Cool completely before serving.