From Brotato Wiki
+15% Piercing Damage. Can't go above base damage
Pumpkin is an in-game item that can be bought from Shop or can be found in a Crate. Most Items increase the Stats of the player while some of them provide unique effects.
Several weapons with Piercing, and the Bandana item grants piercing attacks that have damage falloff for every time the attack pierces a target. Pumpkin reduces the damage falloff by 15%. So if an attack loses 50% of its damage with each pierce, it only loses 35% if you have a pumpkin. Pumpkins can be stacked, removing 15% each. If you have 4 pumpkins, for a total of 60% reduced damage, the 50% damage falloff is completely removed, and the attacks will deal full damage on each piercing/hit.
See Piercing for more info and examples.