https://www.gatestoneinstitute.org/12896/is-the-truth-the-truth-when-it-comes-to
All the Special Counsel needs, in order to charge a subject of an investigation with lying to a prosecutor, is a single witnesses willing to contradict the subject.
The witness may not only be “singing,” he may also be “composing” – that is making up or embellishing a story because he knows that the better his story, the better the deal.
Under federal law, the testimony of such a flipped witness need not be corroborated in order to secure a conviction.
Even one question that results in an answer that is contradicted by one witness would be enough to spring the perjury trap.
When Rudy Giuliani stated, perhaps inartfully, that “truth isn’t truth,” he was getting at a higher – or should I say lower – truth. This is a truth that virtually every experienced defense attorney and prosecutor understands: namely that prosecutors get to pick which witness — and which “truth” — to believe.
Giuliani was discussing President Trump’s decision whether or not to be interviewed by Special Counsel Robert Mueller. Giuliani made the point that even if President Trump testified truthfully, he could be accused of lying to a prosecutor – a serious felony – if the prosecutor chose to believe witnesses who have provided a different account.