you can bring your own phone (GSM unlocked) and just purchase a prepaid SIM card for S/5 or so. You can then add soles whenever you want. The SIM tel # will become deactivated after a year or so depending upon the carrier. The way to keep it active is to ask someone in Peru to add soles to it or use your Peruvian bank account to add soles once and a while.
If you want to buy a prepaid phone in Peru you can do that too there are prepaid accounts that are not monthly ones but rather prepaid ones you can fill up once a while.
Buy the SIM card at an official cell operator, not a concesionario. This will reduce the risk of having your personal information (DNI or passport) sold to someone who wants to use your cel for their own purposes. To find the 'official' operator go to the cell operator's web site and find their main address in the city where you will be. For example Lima has only one official operator.