Citizens of the United States, Canada, United Kingdom, South Africa, New Zealand, and Australia do not require visas to enter Peru as tourists at the present time, nor do residents of any other European or Latin American country. When visitors enter the country, you can get anything from 30 to 180 days stamped into both a passport and an embarkation card that travelers must keep until they exit the country. If you require more than 30 days, be ready to support your argument by explaining your travel plans and showing your return ticket.
Extensions can be arranged at Peru’s immigration offices in Lima [1], Arequipa [2], Cusco [3], Iquitos [4], Puno [5], and Trujillo [6] for US$21. There are also immigration offices on the border checkpoints with Chile, Bolivia (Desaguadero and Yunguyo), and Ecuador, though at this point it is easier just to leave the country, stay the night, and reenter on a fresh visa.
Always make a photocopy of your passport and your return ticket and store it in a separate place. Carry yours in a money belt underneath your clothing, or leave it in a security box at your hotel. If your passport is lost or stolen, your only recourse is to head to your embassy in Lima [1]. If you have lost or had your passport stolen before, it may take up to a week while your embassy runs an international check on your identity.
Links:
[1] http://www.moon.com/destinations/peru/lima
[2] http://www.moon.com/destinations/peru/lake-titicaca-and-canyon-country/arequipa
[3] http://www.moon.com/destinations/peru/cusco
[4] http://www.moon.com/destinations/peru/the-amazon/iquitos
[5] http://www.moon.com/destinations/peru/lake-titicaca-and-canyon-country/puno
[6] http://www.moon.com/destinations/peru/trujillo-and-the-north-coast/trujillo