I am planning a trip for later this year myself. i heard that you should definitely go in the "dry season".
this site explains it best:
Peak Season
Peak travel season for foreigners is in great part determined by weather. Peru experiences two very distinct seasons, wet and dry -- terms that are much more relevant than "summer" and "winter." Peru's high season for travel coincides with the driest months: May through October, with by far the greatest number of visitors in July and August. May and September are particularly fine months to visit much of the country. Airlines and hotels also consider the holiday period mid-December through mid-January as peak season.
From June to September (winter in the Southern Hemisphere) in the highlands, days are clear and often spectacularly sunny, with chilly or downright cold nights, especially at high elevations. For trekking in the mountains, including the Inca Trail, these are by far the best months. This is also the best time of the year to visit the Amazon basin: Mosquitoes are fewer, and many fauna stay close to the rivers (although some people prefer to travel in the jungle during the wet season, when higher water levels allow more river penetration). Note that Peruvians travel in huge numbers around July 28, the national holiday, and finding accommodations in popular destinations around this time can be difficult.