Makkos, Chapter 1, Mishnah 5. This mishnah continues the previous one, and deals with a case in which one pair of witnesses has already been proven to be zomemin: another pair came and told them that they had been together with them at the very time when they claimed the incident took place.
The flow of the mishnah, step by step:
"U'va'u acherim v'hezimum" - another pair of witnesses testifies about that very same act about which the first pair testified: such and such a murder, committed by such and such a person, at such and such a time. And those same two who proved the first witnesses to be zomemin come and do the same to the second pair as well, saying: you were with us at that time, and not in the place where you claimed to have been.
"Ba'u acherim v'hezimum" - yet another pair comes and also testifies about the murder, giving the testimony of what they saw; and once again those same two come and say: you too were not there.
"Afilu me'ah - kulan yehareigu" - even if there are a hundred such pairs, all of them are put to death, all of them receive the death penalty.
The view of Rabbi Yehudah:
Rabbi Yehudah disagrees and says: "Istasis hi zo" - this is a kind of conspiracy, "v'einan neheragos ela kat rishonah bilvad" - only the very first pair is put to death.
As for all the pairs who testified after them about that very same murder, even though that same pair came and claimed that they were liars - we disregard the witnesses who claim they are zomemin, the second set of witnesses, because we assume that they have a conspiracy in hand: to declare a zomem and to prove a liar anyone who comes to testify about that murder and about that murderer.
In summary: According to the Sages, every single pair that is proven to be zomemin is put to death, even a hundred pairs. According to Rabbi Yehudah, only the first group is put to death, since the repeated claims of hazamah about that very same act indicate a conspiracy on the part of those making the claim.