Heat oil in a 12-inch heavy skillet over moderate heat until hot but not smoking, then cook scallions, stirring occasionally, until softened, about 3 minutes.
Add garlic and cook, stirring, 1 minute.
Add corn, zucchini, cumin, salt, and pepper and cook, stirring occasionally, until zucchini is tender, 4 to 6 minutes. Stir in cilantro and season with salt and pepper.