"Perfect change" is what you give a casier so you don't get change back. "A perfect change" is the combination of coins you might carry so you can give perfect change.
Customer: *gives her a five, a one, two quarters, a dime, a nickel, and three pennies.
Cashier: Your change comes to...um...nothing.
Man, I've got a lot of coins. I should probably make a perfect change.