Objection 1. It would seem that it is not always a mortal sin to give false evidence. For a person may happen to give false evidence, through ignorance of fact. Now such ignorance excuses from mortal sin. Therefore the giving of false evidence is not always a mortal sin.
Objection 2. Further, a lie that benefits someone and hurts no man is officious, and this is not a mortal sin. Now sometimes a lie of this kind occurs in false evidence, as when a person gives false evidence in order to save a man from death, or from an unjust sentence which threatens him through other false witnesses or a perverse judge. Therefore in such cases it is not a mortal sin to give false evidence.
Objection 3. Further, a witness is required to take an oath in order that he may fear to commit a mortal sin of perjury. But this would not be necessary, if it were already a mortal sin to give false evidence. Therefore the giving of false evidence is not always mortal sin.
On the contrary, It is written (Proverbs 19:5): "A false witness shall not be unpunished."
I answer that, False evidence has a threefold deformity. The first is owing to perjury, since witnesses are admitted only on oath and on this count it is always a mortal sin. Secondly, owing to the violation of justice, and on this account it is a mortal sin generically, even as any kind of injustice. Hence the prohibition of false evidence by the precept of the decalogue is expressed in this form when it is said (Exodus 20:16), "Thou shalt not bear false witness against thy neighbor." For one does nothing against a man by preventing him from doing someone an injury, but only by taking away his justice. Thirdly, owing to the falsehood itself, by reason of which every lie is a sin: on this account, the giving of false evidence is not always a mortal sin.
Reply to Objection 1. In giving evidence a man ought not to affirm as certain, as though he knew it, that about which he is not certain and he should confess his doubt in doubtful terms, and that which he is certain about, in terms of certainty. Owing however to the frailty of the human memory, a man sometimes thinks he is certain about something that is not true; and then if after thinking over the matter with due care he deems himself certain about that false thing, he does not sin mortally if he asserts it, because the evidence which he gives is not directly an intentionally, but accidentally contrary to what he intends.
Reply to Objection 2. An unjust judgment is not a judgment, wherefore the false evidence given in an unjust judgment, in order to prevent injustice is not a mortal sin by virtue of the judgment, but only by reason of the oath violated.
Reply to Objection 3. Men abhor chiefly those sins that are against God as being most grievous, and among them is perjury: whereas they do not abhor so much sins against their neighbor. Consequently, for the greater certitude of evidence, the witness is required to take a oath.
Source: Summa Theologica (New Advent)