What perk do I need to make my dragon priest mask legendary? Is it enchanting?

smithing

Smithing. Plus consider using an enchanted item (usually in shops) such as necklace of peerless smithing, and a Blacksmiths Elixir. They can be used together, i have tried many things and works well.

Which smiting perk?

The one in the center of the blacksmith constellation, that lets you improve enchanted items.

oh haha sorry I compleatly overlooked the fact that you said which perk lol. you need the Arcane blacksmith perk which is the one in the center of the smithing tree and it would probably help to get the perk for whatever material you need to improve it. ( it should say at the bottom of the screen in the improvements menu)