For a simple dish like this, it is essential that the shrimp be very fresh.
In a small bowl, whisk together rice wine, soy sauce, and sugar, and set aside.
Arrange bok-choy leaves in the two compartments of a bamboo steamer; divide shrimp between the two layers, and cover. Set steamer over a wok filled with 5 to 6 cups of water; place wok over medium heat, and steam until shrimp are cooked through, 8 to 10 minutes. Transfer shrimp to a serving plate, pour the sauce over, and garnish with scallion. Serve immediately.