Let me see if I can explain in simpler terms. This is a pretty classic example that you’d find in a textbook or a class.
Steve is a witness in a civil suit for damages against Becca. A department store is suing Becca for the alleged theft of a necklace. Steve takes an oath sworn by the court commissioner. He thereafter testifies that he saw Becca try on the necklace and then walk out of the store without paying for it. When the store’s attorney asks Steve what he was doing at the store, Marcus responds that he was buying some jewelry as a gift for his wife. In actuality, Steve was shopping for jewelry as a gift for his girlfriend .
Steve has probably not committed perjury here, because Steve’s reason for being at the store really doesn’t have anything much to do with Becca’s theft of the item. It’s testimony to explain how he came to be a witness to the event. Steve’s lie here sucks, but it is not material to the outcome, so Steve is probably not guilty of perjury.
Now change the scenario such that Steve says he did NOT see Becca walk out of the store with the necklace, because he doesn’t want to admit that he was there and buying jewelry for his girlfriend instead of his wife.
Steve’s lie here can introduce doubt that Becca committed the action, so it is material and now Steve has just committed perjury.
These are simplified and it’s civil instead of criminal but the principle is the same.
There is no “some perjury” but there is a question of whether a lie goes to the level of materiality that impacts the outcome.