Perjury can occur even if the person has not been sworn to tell the truth, such as in a courtroom. Merely signing a document under penalty of perjury that contains false statements can be a crime. Signing an income tax return that contains false information is an act of perjury, for example.
In most jurisdictions, the false information has to be material to the issue or affect the outcome for perjury to be a chargeable crime. If the false statement directly affects the results of the case or causes a unjust decision to be made, the person can be charged with perjury.
Because the crime of perjury can cause a miscarriage of justice to occur, it is considered a very serious crime. Under U.S. federal law, for example, perjury is a felony punishable by up to five years in prison.