Perjury is a crime, not an ordinary lie. In a court of law, a person must promise to tell the truth — sometimes with an oath, sometimes by signing a paper. If they then say something they know is not true, that lie is perjury, and the act of telling it is also called perjury. You usually say someone commits perjury. The word belongs to law, news, and formal talk; you would not use it when a child fibs to a parent, or when friends shade a story. The promise has to come first, and it has to be a legal one. In many countries it can mean a fine or even prison, which is why lawyers treat it as a heavy word. People also stretch it a little in angry talk — 'that's perjury!' — but strictly it is the court (or a sworn statement), not every false word.