Rinse the red beans and pearl barley thoroughly. Place them in a soup pot and add enough water to cover the ingredients by about 1 to 1 1/4 inches (2–3 cm). Bring to a boil over high heat, then reduce to medium-low heat.

Simmer until the water level drops to just cover the ingredients. Add the remaining water and continue cooking until the red beans' skins split and the barley becomes tender.

Add the light brown sugar, stir well, and cook for another 5 minutes. Turn off the heat. Let cool to room temperature, then add ice cubes if desired before serving.