Blend lime juice, curry powder, cumin, sugar, ginger, scallions, garlic, and 1 1/4 teaspoons salt in a blender or a food processor. With machine running, add oil in a slow, steady stream to form a paste.
Cut shrimp along back (through shells) using kitchen scissors, and devein. Loosen shells from flesh using your fingers, keeping shells attached. Gently open each shell, and push some curry paste underneath to coat backs; put shells back in place. Cover with plastic wrap, and refrigerate for at least 30 minutes.
Soak 10 wooden skewers in water for 30 minutes. Meanwhile, heat grill to medium-high. Let shrimp stand at room temperature for 15 minutes. Thread 2 shrimp onto each skewer. Grill until shells are charred on one side, about 3 1/2 minutes. Flip, and cook until opaque, about 2 1/2 minutes more.
Shrimp can be marinated for up to 4 hours.