Preheat oven to 425°F. Bring 1 piecrust to room temperature. Roll out and fit into a 9-inch pie plate, letting excess hang over. Line a rimmed baking sheet with foil. Place pie plate on baking sheet.
Toss berries in a bowl with tapioca, sugar, salt and ginger. Transfer to pie shell. Dot fruit with butter. Remove second dough circle from refrigerator and roll out (or unroll packaged dough). Cut a few slits in crust with a sharp paring knife to vent pie.
Whisk egg with 2 tsp. water; brush on edge of pie shell. Lay dough disk on filling. Press edges together and trim excess, leaving 1 inch hanging over. Roll dough edge under itself; crimp. Brush top of crust with egg wash; sprinkle with sugar, if desired.
Bake pie for 10 minutes. Lower oven temperature to 350°F; bake for 45 to 50 minutes. Cool pie on a rack for 1 hour.