Tractate Makkot - Chapter 1 - Mishnah 6 מכות פרק א׳ משנה ו׳

Perjuring witnesses are not to be put to death until [after] the end of the trial. Because the Sadducees say: “[Perjurers were put to death] only after the accused had [actually] been executed, as it says, “ A life for a life” (Deuteronomy 19:21). The [Pharisaic] Sages said to them: “But has not it already been said “You shall do to him as he schemed to do to his fellow” (Deuteronomy 19:19) which implies when his brother is still alive? If so, why does it say “A life for life”? For it might have been that perjurers are liable to be put to death from the moment their testimony had been taken, therefore the Torah states “A life for a life” that is to say that they are not executed until [after] the termination of the trial.