Peru changed the law last year so that everything is now priced in Peruvian Nuevo Soles (PEN). US dollars and Peruvian Nuevo Soles are still fairly interchangeable in hotels and restaurants, but they will probably give you a poor rate. It is important to note that in the country-side, Soles are now the only currency used.
Visa cards are also available in hotels, restaurants and for Cash Withdrawals but do take about at least 2% fee.
So it is recommend that travelers bring in at least $500 (in US dollars) cash each, then change into PEN at the change house in the airport arrivals and ask the teller to explain what a false Sol note looks like. You can then use your debit card to access dollars from your US account and change into PEN in change-houses as required. Change houses are “Casas de Cambio” in Spanish.