Scripts triggered from monster drops.

Is it possible to trigger a script when picking up an item dropped by a dead monster?

In ZDoom, I suppose? Yes. But it's not trivial because you have to make a custom monster variant that'll drop a custom item. There's no way to have a monster give an item special to what it drops when it dies. So you've got to make the item run the script itself (use a CustomInventory so it can run instructions when picked up), and you'll need a custom monster so that it drops this custom item.

Or define a spawn state with a Thing_SetSpecial action for the dropped item.

What about:

Don't make the monster drop the item, but place the item somewhere outside the map, and have Thing Action 80 set in the effects tab for the item. Then, teleport the item to the monster's corpse when it dies, with thing_move in a script. Then you can pick up the item and the script will activate?

Ten times as complicated - and it doesn't properly do the drop effect.