Part of my fuel cap fell in my fuel tank. How serious is this?

That looks like an Anti-Siphon sleeve not part of the cap. It just keeps someone from sticking a tube in your tank to steal fuel. If the tabs that hold it to the neck of your tank broke, it probably just fell to the bottom of the tank.

 

If the parts are bigger then your finger it shouldn’t fit in the tube and plug a fuel line. But sooner than later you should get it out. Vibrations might eventually wear a hole in the tank or the draw tube, if it stays in one spot for too long. If you can see it, try using your pin puller and another stick to bring it to the top and have a pair of pliers (tie a string on them so you don’t add to the problem) handy to pull it out. Other wise it will be a drain and remove the tank job to get it out.

 

The item sitting in fuel won't "rust" and ruin the engine or tank. It may eventually block the fuel pick-up or drain. Next time you are in for service have mechanics fish it out of tank.

Or you can get a grabber from Lowes, Home Depot, Walmart, auto parts store, or Amazon.

Here is a link to see what you are looking for or use a straightened coat hanger.

CRAFTSMAN CMMT14112 Flexclaw Pickup Tool https://a.co/d/729CtJr

 

question is, how did it break?

someone try to syphon the tank?

 

If its a made from a metal that will stick to a magnet get a telescoping stick magnet at a tool store and fish it out with that. May need to park truck or jack it up in such a way to get that object to move closer to the filler hole.
An old thin fiberglass handled golf club works good too to pull the object along the bottom to where you can see it.

 

Chances are it's made out of aluminum. In that case, you'll need a aluminum magnet, and those things are really hard to find....

 

No issue at all. It will not make any difference, except it may now be easier to fuel/