IF, the internal tubes have corroded within the tank that would be relatively serious. The interesting piece is, how would they corrode. As far as I know, almost ALL the fuel tanks on modern bikes use copper for this overflow. It runs from the lip above the filler point, down through the tank to a nipple on the underside of the tank (or in the case of Kawasaki's) to a nipple under the front of the seat on the top side of the tank towards the rear.
Here is a standard Honda Fuel Injected tank design cutaway. This is for a CBR, post '06.
You can see the two tubes running through the tank. The larger one is the overflow and it's made of a copper and nickel alloy.
The question is, how can it corrode?
It has some of the best general resistance to aqueous corrosion of most of the alloys. It also, when used in a fuel tank, has an epoxy coating....as does the rest of the fuel tank.
After thinking about this for a minute, the idea that the tank coating has been stripped out of the tank keeps recurring to me. Those tubes just don't corrode with the epoxy lining, nor does the tank. It's steel and would turn into a shell of rust and post-apocalyptic dysphoria pretty quick. Imagine hitting the dew point everyday with a low fuel level and lots of atmosphere inside the tank. Water droplets, tiny ones, form on the inner tank surface and then you get rust.
Therefore, if I'm right, the tank has been stripped using that standard commendation, with a water/acid mixture. It removed the epoxy coating off the copper. The copper became exposed to oxygen and began to....that's right...oxidize and degrade over time.
If I am correct, there will be rust on the inner tank surface as well. If there is no rust on the inner tank surface then there is a good chance I'm incorrect on how this happened.
Buy a new tank. You can't plug those nipples. If the line internally is broken and gas from inside the tank can exit that broken copper line, you fuel is essentially exposed to the atmosphere and whatever sparks, fire or any other ignition source the rest of the world is exposed to. Think static electricity here, with a low amount of fuel and lots of airspace in the tank. It would create a might fine explosion worthy of any virul youtube video. The exit of that line is near an exhaust pipe, right?
Throw it way, get a used one on Ebay from a seller with a good rep. Sorry for the bad news. 99% non fixable.