How do I use a five star gold frag in puzzle and survival?

You can sometimes purchase them in special sales or win them as rewards for competing in certain events

To use the 5 star hero perfect frag you need to go to the heroes screen for the hero you want to use it for. (You cannot use it on Samuel, Angelo, Leah, Dara, Hanyu Ichiro, Ironclaw, Eve).

On the hero screen go to the Enhance option and assuming you are not able to enhance there will be an 'obtain' button at the bottom and to the right of it an icon for the frags (if you have some). Tap on that icon and then you can exchange the hero perfect frags for that specific hero.