It is pretty dodgy, and I would not be out much after dark. If you do, use a proper taxi to get back to your hotel.
As another poster said, leave your jewellery and expensive kit at home, preferably the UK.
I don't remember any memorable meals in Peru, least of all Lima. Apart from a plate of ceviche, which was a pound of slices of various raw fish with lemon/lime juice squeezed over. Not memorable for being good though.
The sights in Peru are great, but as you I've suggested above the going is not always easy...