Tosfos (Makos 4b, DH v'Rabanan) writes that when the Gemara on 2b states that Lo Sa'aneh is a Lav without an action, this means that if not for the verse "v'Hitzdiku" we would have said that it is a Lav without an action. However, after the Torah has revealed to us the Din of "v'Hitzdiku," we know that for giving false testimony one does receive Malkus, as the Gemara (2b) derives that if two witnesses said that a Tzadik was in fact a Rasha and then two more come and say that he is a Tzadik, the first two witnesses receive lashes for their false testimony. One learns from this that Malkus is given for false testimony, and, therefore, according to the conclusion of the Gemara, Lo Sa'aneh is considered a Lav with an action. This explains why, according to Tosfos in Yevamos (31b, DH d'Chazu) in the name of Rabeinu Tam that one can send testimony to Beis Din in writing, one receives Malkus for false testimony.

Kol Tuv,

Dovid Bloom