No real life examples can prove that this saying is true. However, they can at least suggest that it is true for some people.
If we wanted to cite examples of absolute power corrupting absolutely, we could point to dictators like Hitler, Stalin, and Mao who did things that harmed their people tremendously. They seemed to have been corrupted by their power. But can we prove that they were? No. It may be that they were corrupt before they gained power. It could be that their corrupt natures caused them to seek absolute power. In that case, corruption would help cause absolute power, not the other way around.
This saying certainly appears to be true. This is shown by everything from bribery cases to evil dictators. But it is not true for all people and we cannot prove that it is ever true.