Do you want to travel to Peru and your question is what is the currency they use? Well, In Peru and Cusco the official currency used is EL SOL.
In many places such as restaurants, Spanish schools, nightclubs, markets it is possible to pay by credit card or debit card. But in other smaller places such as: small shops, traditional markets, cafes; You will need to carry cash in Soles.
A recommendation is to always have small coins, for example if you want to buy a soda and you pay with a bill of 100 Soles they will not receive you. Bills of 10 and 20 soles is recommended.
Withdrawing money in Cusco is not very complicated because there are many banks and ATMs near the center of the city, specifically on Av. El Sol. A very busy and quite commercial avenue. You can withdraw money from the bank in soles, although the exchange rate varies a bit.
Another option is to withdraw the money in dollars from the bank and then you can change it to soles in authorized exchange houses that are located in the El Sol Avenue, between the first and second blocks where the exchange rate is better.
Enjoy Cusco, a safe and majestic city. Walk the streets without worries and just concentrate on the beauty of this place. Good trip.