A 19% impuesto general a las ventas (general sales tax) is levied on everything except goods bought from open-air markets and street vendors. It's usually included in the advertised price and should be included with food and drink. If a business offers you a discount for paying in cash, it probably means they aren't charging sales tax (and not reporting the transaction to the government).
By law restaurants must publish their prices—including taxes and sometimes a 10% service charge—but they do not always do so. They're also prone to levy a cover charge for anything from live entertainment to serving you a roll with your meal. Hotel bills may also add taxes and a 10% service charge.
Departure taxes at Lima's Aeropuerto Internacional Jorge Chávez are $31 USD for international flights and $6.82 USD for domestic flights—all airlines now include the tax in their ticket prices.