Paper Tube (2) drops from the Wall of Flesh and Mechanical Bosses.
Using them in your inventory (the same way as if you would summon bosses) will unlock a random Hardmode accessory for you to buy from the Tinkerer's shop. Though, it will not unlock Post-Plantera accessories, despite them being available in Hardmode. For that, you'll need the next tier of Paper Tube.
Look at all the existing Paper Tubes.