That issue of perjury—proving in a court of law that someone lied—is a difficult thing to prove, even on a clear day. It's one of those things that, for evidentiary purposes, is simply hard to show: that the person accused of lying knew that what he was saying at the time he said it was untrue and intentionally, under oath, uttered an untruth.
La preuve de parjure — c'est-à-dire, la preuve devant les tribunaux qu'une personne a menti — est difficile à établir, et ce, même dans des circonstances favorables.