What to wear in Peru?

It depends on the regions you visit or the time of the year. But in general terms if you visit several different places in Peru, be prepared for different types of climates. For example, on the coastal regions, it can be between mild / hot and humid from December to March/April and moist and cold the rest of the year. This cooler weather is caused by the Humboldt sea current that flows from southern latitudes towards the equatorial line.
If you visit higher ground regions (The Andes), be prepared for big temperature contrasts between day and night time, so a good Polar jacket is always a good idea to bring along with you, as well as a Rain jacket or poncho.
If the Amazon basin is on your list of places to visit in Peru, be prepared for very hot and moist weather, where you will tend to sweat a lot, so several changes of socks or T Shirts should be needed (same number as the days you will spend in the jungle. You should also have with you a long sleeved shirt to avoid some mosquito bites.

In general terms, bring along with you comfortable clothing for long walks, good trekking shoes, a jacket or polar jumper for those cooler nights, a rain coat (thin and not for very cold weather).