Not sure this is in answer to your question, but there are currency exchange counters always open at LIM. As everywhere, I imagine the rate s*cks, but never done it. Thinking what I'd do? There are stores, probably I'd pop into a travel store in LIM and get something I can use rather than suffer bad exchange rates. Yes, in a pinch you can use dollares, everyone knows the exchange rate, but do negotiate it ahead of time. In general, lots of guys in vests standing at corners to exchange dollares/soles in either direction, at good rates.