Thoroughly rinse the squid under cold water and pat dry. Cut the tentacles in half lengthwise. Cut tubes in half lengthwise and then into 1/2-inch wide pieces. Set aside.
Over high heat, bring 1/2 inch of water to a boil in a 6-quart saucepan. Decrease the heat in order to maintain a simmer. Place the squid in a steamer basket and gently set over the simmering water. Cover and steam for 2 to 4 minutes. Remove the steamer basket from the pot and plunge the squid into ice water to stop the cooking. Remove from the water and drain thoroughly. Set aside.
In a large mixing bowl, whisk together the lemon juice, olive oil, salt, black pepper, cumin, and cayenne pepper. Add the squid, tomato, onion, capers, and cilantro and toss until combined. Cover and refrigerate for 1 hour before serving.