The Talmud tells us that truth is the seal of God (Sanhedrin 64a) and He warns us in the Torah to distance ourselves from every false matter (Exodus 23:7). Truth is an important foundation of society and it should be embraced by a person. To lie under oath is utterly reprehensible and undermines the entire structure of the justice system. If we can’t rely on witnesses, all sorts of chaos can happen, from property being seized from its rightful owners to innocent people being executed. It’s bad news.
The details of this law include what testimony includes, how it is given, who may serve as a witness for what types of cases, etc. This law applies to men but not to women, since women do not serve as witnesses under Jewish law. (This is not to suggest that women are unreliable or that they can’t be trusted. If that were the case, no man could eat in his own home or engage in marital relations, since he relies on his wife to maintain and tell him about matters of kosher food and family purity.)
The prohibition against perjury is #285 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #39 of the 194 negative mitzvos that can be fulfilled today in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar. In the Talmud, it is discussed in the second and third chapters of Sanhedrin; in the Shulchan Aruch, it is found starting in Choshen Mishpat 28.