A caveat about making the perfect IV codes permanent: this would also make the game harder, because the code affects not just wild and hatched pokémon, but also trainer pokémon and even lair pokémon. With the code permanently on every pokémon you battle will hit you faster and harder while taking less damage and being able to soak more of it to boot. Not only this nullifies any advantage you could get by choosing (or breeding) good pokémon, perfect IVs make more of a difference for trainer and wild pokémon because those aren't properly EV trained.Because you don't hold it when the egg hatches. An eggs stats are determined when the NPC has the egg ready for you. So you need to be holding it (or have it activated) when the egg becomes ready for pickup.