Peru and Argentina are very far apart, so it would be a pretty long cruise that hits both. There are many that go from Buenos Aires to Chile, and more that start in Rio and also visit Argentina and Chile
I like to shop on vacationstogo.com -- they list cruises by destination, so it's easy to see every South American cruise for the next year or so. Remember to look at Repositioning cruises too ... you might find one starting in Buenos Aires, going around the cape, and up to Florida.