Although I agree with these posts, you can feed them beef! However, It is very fatty and not something you want to feed daily but as a once in a while treat it would be fine! You should also only feed them organic beef, as regular store bought beef has many different chemicals and steroids that could harm the fish. You can feed them mice but they should be dead and gutted before feeding to the fish. I know many farmers from South America lose young cows, sheep, young horses, chickens, ducks etc. to piranha all the time! So they do eat them in the wild but it is not an everyday occurance and as TKOS said doing this in a tank can and will cause major water problems! As they dont have the water flow that the river the fish live in has.
So although it is okay to do once in a while I would also recommend sticking to home grown feeder fish!