As I mentioned, dollars are not accepted as payment in Europe. However, in Peru, they are accepted widely as well as their own currency, the Nuevo Sol. However, when taking bills in Peru, they must be in perfect condition with no rips, tears, extreme wear, or deep folds.
The first thing every traveler should know is the exchange rate of the country they are visiting. I like this site to get the most current rate: http://finance.yahoo.com/currency-converter/#from=USD;to=EUR;amt=1
The worst exchange rate is at the the exchange booths at the airport or in the middle of cities. Avoid them. Travelers checks are also passe and are of little or no use.
Whether traveling in Peru or Europe, the best place to get cash is at an ATM. The exchange fees are the lowest Make sure to alert the bank of travel plans beforehand though.
It is possible to pick up currency before departing from a major bank like Bank of America, Citibank, or Chase. However, the exchange rate will probably not be as favorable as using an ATM in the country you are visiting. BTW, most Peruvian ATM's distribute dollars as well as sols.
When using a bank card to pay or withdraw money, get a card that has no foreign transaction fees. That 3% they commonly charge can add up.
And my last trick for getting foreign currency is to be the person collecting the cash at a group dinner, then pay the bill with a credit card.