My firm takes lots of depositions–you know, the proceeding with a court reporter but no judge–virtually always in federal court cases where our client is a defendant. In the last year (no way I am going to tell you who, when, or where) we conducted one where the witness in a business case lied under oath–lied and lied repeatedly–about critical material facts and we can prove it. Our client has been informed. The question is: should we pursue it and use it. Perjury is the “willful and corrupt taking of a false oath in regard to a material matter in a judicial proceeding”. It is sometimes called “lying under oath”; that is, deliberately telling a lie in a courtroom proceeding after having taken an oath to tell the truth. It is important that the false statement be material to the case at hand””that it Could affect the outcome of the case. It is not considered perjury, for example, to lie about your age, unless your age is a key factor in proving the case.
In criminal cases, perjury can be used as a threat. Although it is a very serious crime under state and Federal laws, and while prosecutors often threaten prosecution, the number of actual prosecutions for perjury is tiny.
Perjury prosecutions stemming from civil lawsuits are particularly rare. This is because it is difficult to prove that someone is intentionally misstating a material fact, rather than simply testifying honestly from faulty memory.