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?

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

What you are looking for is ItemStack#shrink

Other methods like grow and setCount should work as well.