[1.16.4] Remove 1 item in offhand ItemStack when an event fires.

[Moomallowz]

Here's my event, which fires when a fishing rod is cast (or reeled in, I'll fix that later).
 

@SubscribeEvent
public static void usedWorm(PlayerInteractEvent.RightClickItem event) {
    LivingEntity player = event.getEntityLiving();
    if (player.getHeldItemOffhand().getItem() == RegistryHandler.WORM.get()) {
        if (player.getHeldItemMainhand().getItem() == Items.FISHING_ROD) {
            player.addPotionEffect(new EffectInstance(Effects.LUCK, 2400));
            //fairly certain the ItemStack thingy goes here, not sure about syntax though
        }
    }
}

 

The event is currently allowing the offhand item (the Worm) to be used an infinite number of times.
To maintain balance, a Worm must be consumed when this is done. It is absolutely imperative to catching funny fish.
I am unfamiliar with ItemStacks and their applications. Is it possible to dumb this down enough for my ogre brain to comprehend?

[Moomallowz]

Happy New Year to anyone who stumbles across this topic, by the way.

[poopoodice]

What you are looking for is ItemStack#shrink

Other methods like grow and setCount should work as well.