I seriously don't know the letter of the law but you will not have any problems. We've hauled about every size knife there is into Peru and many of them at one time without any problems. If they ask you anything or get sticky just tell them you are going to the jungle and the knives are for some of the military guys that are going with you. That always works.
The only thing I ever got in an argument about was my camera gear. The little guy just past the green and red customs button said I needed to pay a tax because it was "professional" gear. I asked him to take me to his supervisor. She was extremely nice and jumped the little guy's ass, especially when I produced a business card of the head of the NP and explained I was down there to work with them.