Step 1Roast perilla seeds in small dry skillet over medium-low heat, shaking skillet, until golden, 8 to 10 minutes. Set aside.
Step 2Wash beans, then soak in hot water until plumped and shells have begun to come off, at least 2 hours. Rinse and remove shells. Place beans in large pot and add water to cover about 1 inch above beans. Bring to boil. Reduce heat and simmer 35 to 40 minutes. Occasionally skim foam from top. Remove beans from heat, rinse thoroughly in cold water, then drain.
Step 3Puree seeds then combine with beans. Puree bean mixture in batches in food processor with water so that you process 1 cup beans with 2 cups water, until liquefied. Strain liquid through cheesecloth or fine mesh strainer. Add salt to taste.
Step 4Refrigerate soy liquid 4 to 5 hours until thoroughly cool or chill over a bowl of ice water 1 hour.
Step 5Bring 6 to 8 cups water to boil in large saucepan. Loosen bundles of somen and add to water. Boil until tender, 2 to 3 minutes. Remove from heat and rinse thoroughly in cold water. Drain.
Step 6Divide noodles among 8 to 10 large bowls. Carefully pour cold soy soup over noodles. Garnish with cucumbers and 1 to 2 tomato wedges per bowl and sprinkle each with black sesame seeds.