The opinion of the Rambam [Eidus 20-2] is that the פטור of "כאשר זמם ולא כאשר עשה" - the עדים זוממים are only punished when they ATTEMPTED to impose a punishment but not when the punishment was actually carried out, only applies to נפשות, capital cases. But when it comes to זממה דמלקות - when the falsely accused was given מלקות, there is no פטור and the עדים זוממין receive מלקות. The Raavad sharply disagrees.
The Chasam Sofer [חו"מ סי' ל"ג] explains the opinion of the Rambam based on the Magid Mishna [פ"ב ממאכלות אסורות ה"א] who wrote that in a place where there is an explicit לאו הבא מכלל עשה [an aveirah derived from a positively framed mitzva], we may now derive punishment from a קל וחומר [even though normally the rule is "אין עונשין מן הדין" - punishment may not be meted out on the basis of a קל וחומר]. That is what the Rambam means: Even though לא תענה is a לאו שאין בו מעשה and therefore doesn't mandate מלקות, nevertheless, once we already have "לא תענה" we can then learn from a קל וחומר in order to give מלקות, just like a לאו הבא מכלל עשה allows us to learn a punishment from a קל וחומר. The קל וחומר goes as follows: If there is מלקות when the עדים merely ATTEMPTED to have the person punished, most certainly there is מלקות when the עדים actually brought about מלקות in practice. And like we said, the קל וחומר cannot be rejected by saying אין עונשין מן הדין, because there is a לאו of לא תענה [forbidding false testimony] that allows us to go forward and derive a punishment from a קל וחומר. However, when it comes to דיני נפשות - capital punishment, the rule of the Magid Mishna doesn't apply [why not? I don't know! E.E.]. So now we understand why the Rambam says that even if מלקות were already given, the עדים זוממין receive מלקות, but the same doesn't apply to דיני נפשות. In the words of the Chasam Sofer:
Based on what we wrote, we may question this explanation of the Chasam Sofer. The M"M was talking about a case where both the קל וחומר and the עשה related to the identical איסור. In such a case the M"M was מחדש that it is enough that there is an איסור עשה on order to make it possible to learn punishment further from a קל וחומר. But with respect to עדים זוממין, the לאו and the קל וחומר are not relating to the same איסור. The קל וחומר relates to the punishment of כאשר זמם that is not coming as a punishment for the איסור but rather המשכת החיוב - the drawing of the obligation of the falsely accused on to the accuser [as we explained]. While the לאו relates to the עבירה of false testimony ["לא תענה"]. In such an instance we cannot combine the קל וחומר and איסור to give a punishment and we return to the rule of "אין עונשין מן הדין" - We cannot punish based on a קל וחומר.
However for this very reason the opinion of the Rambam [that we give מלקות to the עדים זוממין even after the accused received מלקות and we don't say "כאשר זמם - ולא כאשר עשה"] makes perfect sense and we can't ask on him that "אין עונשין מן הדין" [which would be the only way to give the מלקות, as we explained - If they get מלקות in a case where the accused had not yet received מלקות, certainly in a case where he did]. Since he holds that the punishment of an עד זומם is unique and anomalous among all other punishments insofar as that it is not merely punishment for evil acts but המשכת החיוב, he can also maintain that the general rule of "אין עונשין מן הדין" doesn't apply to it.
However for this very reason the opinion of the Rambam [that we give מלקות to the עדים זוממין even after the accused received מלקות and we don't say "כאשר זמם - ולא כאשר עשה"] makes perfect sense and we can't ask on him that "אין עונשין מן הדין" [which would be the only way to give the מלקות, as we explained - If they get מלקות in a case where the accused had not yet received מלקות, certainly in a case where he did]. Since he holds that the punishment of an עד זומם is unique and anomalous among all other punishments insofar as that it is not merely punishment for evil acts but המשכת החיוב, he can also maintain that the general rule of "אין עונשין מן הדין" doesn't apply to it.
