Cook the acini de pepe according to package directions until al dente. Drain, rinse under cold water to stop cooking, and drain again. Set aside.

In a large bowl, combine the cooked and drained pasta with the drained crushed pineapple, mandarin oranges, and halved maraschino cherries.

Reserve 2 cups total of the drained fruit juices (from pineapple, mandarins, and cherries). In a medium saucepan, stir together the 2 cups reserved juice and 3/4 cup sugar. Heat over medium until it comes to a full, rolling boil.

Temper the beaten egg by slowly whisking 2 tablespoons of the hot juice into the egg to warm it, then pour the tempered egg back into the boiling juice while whisking rapidly.

Make a cornstarch slurry by mixing 2 tablespoons cold water with 2 tablespoons cornstarch until smooth. Add the slurry to the boiling juice and whisk constantly. Continue cooking and whisking until the mixture thickens and becomes glossy (about 1–2 minutes after returning to a boil).

Remove the pot from heat. Pour the thickened juice over the bowl with pasta and fruit. Stir gently to coat everything evenly.

Cover and refrigerate for several hours until the mixture is chilled and slightly set — overnight is ideal.

Just before serving, fold the thawed Cool Whip into the chilled pasta-and-fruit mixture until combined. Chill again briefly if needed, then serve.