How do charges get filed against someone who has lied under oath?

Lying under oath is perjury, which is a crime under California Penal Code §118. Only the District Attorney can file a criminal case. You could file a police report or contact the DA's office; however, most prosecutors will only file perjury charges that are very easy to prove. For instance, if someone fails to disclose a criminal conviction on an application for welfare benefits, which clearly states that is is signed under penalty of perjury, it is very easy to present the court documents that prove the person was convicted. Penal Code §118 also says that a person cannot be convicted of perjury based on the testimony of just one person who says they lied. There must be some other evidence to prove the sworn statement was false.