I assume you will be flying into Lima, then on to Iquitos or Tarapoto for the Amazon? Take any knife you wish to take in your checked luggage, blade length doesn't matter. If by odd chance Customs does say anything just explain you're going to the jungle and they will let you pass on through. The only hassle I have ever had was the last trip in June. Delta Airlines freaked over all the big knives and solar chargers in my checked luggage (of course solar chargers with sealed battery packs do look like some type of improvised bomb). I got pulled and searched but after I explained everything they let the bag pass.
When are you going to be there? We are headed back at the end of September and again in October with clients for the Air Force Survival School.
Jeff