INTRODUCTION: Peru can be visited all year round, but it all depends what do you want to do or visit.

The country covers three major geographic and climatic zones. It is split north to south by the Andes Mountains and is covered by the Amazon Jungle in the east. The country generally experiences two seasons: a wet season from November through March and a dry season from April through October. Most leisure travelers come to Peru from June through September during the dry season, which is the best time Machu Picchu because the sky is clear. The dry season is also the best time to visit the jungle. For visits to the coast, however, the best time is November through March when the weather is hot and perfect for the beach. The country sees peaks in demand during the Christmas and Easter holidays. The city of Cuzco attracts many visitors for the week-long festivities of Inti Raymi in June.

Summer is from December to March but, it is the rainy season on the highlands and the jungle. During February the Inca Trail is closed for tourists. In summer the beaches south of Lima are great. You can also go to the North of Peru and visit Mancora all year round, as it is close to the Equator and will be sunny.
It never rains in Lima, but it gets cold due to the humidity between June and August. Its normally overcast with fog. If you go out of Lima towards the central mountains, you will find the sun about an hour away.
Most popular time to visit Cusco is between June and August, but the recommend time to visit is in April, May or September to October, just after and before the rainy season and the tourist crowds. If you do not mind the rain and are not doing the Inca trail, the highlands are beautiful green and flowery during the rainy season.
The sun is always out in the highlands, but it will be chilly in the afternoons and quite cold during winter at night. Dress  with clothing in layers ( t shirt, long sleeve shirt, sweater, jacket) so you can "peel" them off as it gets warmer and put them back on when it gets colder. Bring a small backpack to carry your clothing and tourist apparel.

Useful Links about Weather in Peru

Herein you have some good links that will help you to check the Weather in Peru.

Senamhi     -   Weather2travel      -  Weather Underground - Weather channel

Weather by Regions of Peru

The Mountain Highlands (Cusco, Lake Titicaca, etc)

Mid April - October. 
This period is the dry season, with hot, dry days and cold, dry nights, often hovering just above freezing, particularly in June and July. May is perhaps the best month with the countryside exceptionally lush, yet with superb views and fine weather. You will find the flowers in full bloom, the grass green and the streams full. 
Perus high season is from June to August which coincides with the dry season and summer holidays in North America and Europe. You will find Cusco a pretty cosmopolitan city with tourists from all over the globe converging on Machu Picchu.

November - Mid April 
This is the wet season with most rain in January and February. It is usually clear and dry most mornings with outbursts of heavy rain in the afternoons. The daily temperatures are typically mild with only a small drop at night.
 
The Inca Trail is much less crowded during this period and there is a more abundant fresh water supply, but of course be well equipped for the rain.  You will also find some roads may become impassable particularly when trying to visit villages off the beaten track. 
Many of Perus major festivals such as Carnival and Easter Week take place during this period.

The Coast  (Lima, Nasca, Arequipa)

December - April 
This is summertime on the coast where the weather is hot and dry and ideal for swimming and getting a tan. Temperatures on average range from 25 - 35C. There is little or no rain during these months. The beaches around Lima and the North can packed during the months of January and February which coincide with school holidays.

May - November 
From May to November the temperature drops a bit and you will find blankets of sea mist engulfing the coast from the south right up to about 200 km north of Lima. At this time of year only the northern beaches such as Mancora and Punta Sal are warm enough to provide pleasant swimming.

The Jungle and Amazon

April - October 
This is the dry season with daily temperatures averaging 30 -35C. However cold fronts from the South Atlantic are common when the temperatures can drop to 15C during the day and 13C at night. 
The dry season is the best time to visit the jungle regions, there are fewer mosquitoes and the rivers are low, exposing the beaches. It is also a good time to see nesting and to view the animals at close range, as they stay close to the rivers and are more easily seen.

November - March 
This is the wet season, hot and humid, when you can expect heavy rain at anytime. It only rains for a few hours at a time, so it is not enough to spoil your trip. Wellington boots are a must though, as some of the jungle trails can become small rivers.

Article about Weather in Peru

Weather During High-Season

Oddly enough, your summer vacation is actually winter in Cusco.  So you're wondering what it feels like.  Well everyone says it's cold.  What kind of cold?

I'm from Ontario, Canada.  For those of you who think that means my standard is super cold and we live in igloos.... we don't.  Our winters feels similar to the winters in New York.

I can tell you June/July in Cusco feels like October-November.  It is cold enough for a padded jacket.  What I mean by that is not a winter jacket, but a thinner winterish jacket.  Not a spring or fall jacket or wind breaker, but a little bit thicker. 

You'll feel pretty cold walking around, but it's not like your cheeks or hands get cold. 

Your hotel blankets at night will be cold to the touch, so it is better to have socks and maybe a hoodie and sweat pants to sleep in.

It is best to book a hotel that has a heater if you want to get good sleep.  Add to the altitude sickness symptom of sleeplessness of being cold at night - its not a fun mix.  Most hotels in Cusco and the Sacred Valley don't have heated rooms, but they may have portable plug-in heaters they may give you for free or rent to you for about 10 soles.  So definitely check that out.

TIPS:

-pack a Fall season jacket

-pack socks

-pack sweat pants to sleep in

-pack a hoodie

-before booking in June/July, ask you hotel if they have portable heaters

PACKING FOR PERU : Jungle, Andes and the city of Lima

The best clothing for the jungle was long pants, the best thing is very thin cotton (or linen) drawstring pants, like pajama pants or thin surgical scrub pants.

Avoid wearing dark clothes in the jungle--dark colors attract bugs

For the city I have some synthetic tops that wash and dry quickly and these will work in the Andes, also. Fleece is the best outer covering because it is so darn light. A couple pair of jeans, two pair of dress slacks, one pair of cropped pants

Note: the DEET concentratoin tells you how long the DEET lasts. For example, 100% DEET will last about 10 hours; 10% DEET about 90 minutes, 25% DEET 2.5 hours, etc. Also, the majority of the DEET product, when sprayed onto the skin, either evaporates or is absorbed into the body,which is why such high concentrations are required.

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