It's common practice for anyone called to testify before congress to request a closed session if they believe their testimony will violate their oath of secrecy. Clapper did not. Instead he testified before open congress and lied, which was unnecessary, unless he was guilty of the charges in question -- knowingly directing NSA staff to spy on Americans, which the NSA's charter explicitly disallows.

Therefore Clapper deliberately misled authorities in order to escape prosecution for a crime.

That is the very definition of perjury.