What will happen in case of an outward decrease in demand under normal downward sloping demand curve and perfectly elastic supply curve of a commodity?

When the supply curve is perfectly elastic, it is horizontal — meaning sellers are willing to supply any quantity at a fixed price. If demand decreases (shifts leftward) under a normal downward sloping demand curve, the new intersection with the unchanged horizontal supply curve occurs at the same price but at a lower quantity. Hence the equilibrium quantity decreases while the equilibrium price remains unchanged.