Bring a large pot of water, add salt, and cook the orzo according to the package directions until it is just al dente. Before draining, reserve about a cup of pasta water.
8 ounces (about 1⅓ cups) orzo
Drain the orzo and return it immediately to the warm pasta pot. Add about ¼ cup of pesto the orzo and stir it until the pasta is evenly coated. If your pesto is thick, add some of the pasta water about a tablespoon at at time to thin it slightly.
¼ cup pesto
Transfer the orzo to a glass bowl and let cool to room temperature.
When cool, add the perlini mozzarella, chopped tomatoes, chopped olives, salt, and pepper, and toss gently to mix.
½ cup perlini mozzarella, 10 cherry or grape tomatoes, halved or quartered if large, 10 pitted Kalamata olives, sliced, ¼ teaspoon salt, ⅛ teaspoon black pepper
Garnish with toasted pine nuts and serve at room temperature.
2 tablespoons toasted pine nuts