It's easy to become paranoid and decide it's best to stay at home. Peru recognizes that tourism plays an important part in its developing economy and has taken great steps in the last few years to change its poor security record. You'll find a lot more police, especially plain clothed officers, in the towns and cities most frequently visited by tourists.
Personal security is a very subjective thing to talk about.
At the end of the day you need to be careful and use your common-sense. Thankfully the instances of assaults on tourists are very rare and, nine times out of ten, rarer than in your own country. Never take a taxi waiting outside the airport grounds. When traveling from your hotel to the airport, go with a taxi recommended by the hotel. Try not to arrive in a new city or town late at night.