Hello, are you referring to the hole shown to the left of the gas fill opening in this picture? With the red box around it.
If so that is there on purpose. It is a drain hole. Mainly, it is to let water drain out since the gas door is not sealed. But the other purpose is to let any fuel spilled or leaking from the gas cap/fill opening to run out of the drain instead of pooling up and running down the side of the car. This hole should not be plugged up.
If there is fuel leaking on the ground, going through this hole and down the drain hose, then the hole is not the problem. The problem is the fuel getting to the hole!
One very likely cause is that you may have a leaky gas cap. Consider replacing the gas cap seal as shown here:
Mercedes W123 Fuel Cap and Oil Cap Seal Replacement
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