Perjure, forswear are comparable when they mean to violate one’s oath or, when used reflexively, to make a false swearer of oneself.
In general use perjure is often employed less precisely than in law, where it is a technical term meaning to make a willfully false statement of fact or sometimes of an intention to do something, while under oath or under a solemn affirmation to tell the truth. In general use perjure often implies making a liar of oneself whether one is under oath or not.
Forswear (see also ABJURE ) often implies a violation of an oath, promise, or vow, but it may also suggest untruth or ill faith to something (as one’s principles, one’s beliefs, or the laws of one’s country) as sacred as an oath.