What is a 5* perfect frag used for exactly?

The Hero Perfect frags can be exchanged for frags for some of the 5* heroes (but not all). If you click on them in your 'bag' under 'Gadget' you will see a list they CAN'T be used for.

To use them go to your hero menu and select any other 5* hero. e.g. Ephriam. Go to the Enhance tab and next to the button at the bottom you will see any hero perfect frags that you have. Tap on this and you have the option to exchange them for frags for your selected hero. If you don't see the hero perfect frags option then either you don't have any or the hero is not one of the accepted ones.

Perfect frags come along with some purchases and possibly can be dropped with prizes. I got 10 of them when I bought a package but I haven't yet seen them drop from any game play.