Falsehood, in fact, means
telling a lie which denotes the state of not telling the truth;
in other words, it means telling of a thing opposite of what it
is in reality knowingly and stubbornly to achieve a substantial
or social or psychological aim and the like.
Perjury is concerned with a person who bears witness to what he doesn't know or counter to what is true. It is the ugliest and the most abominable sin in Allah's sight and as a proof to that Allah associated that to what the ancient people used to worship of idols when He said: "So, shun the abomination of (worshipping) of idol and shun lying speech (false statements)."(Al-Haj 30)
The difference between slander (telling false statement) whether inside or outside the court of law or "perjury" which strictly means telling a false statement especially in a court of law after promising to tell the truth which is usually associated with the judiciary and other sources which demand giving evidence as an eye-witness.
In the hadith came that the Prophet (P.B.U.H.) said: "Would you like me to tell you who is the worst in committing sins? We all replied in one voice: " Yes, Allah's Messenger. "The Messenger replied saying: "Joining an associate with Allah, showing ingratitude to parents; He mentioned these two reasons while was leaning against a pillow and then added saying:" False swearing, false swearing and he remained repeating it several times until we said" How good of him would it be if he kept silent." The Messenger (P.B.U.H.)
This repetition of avoiding saying or swearing falsehood shows the importance of the hadith which one wants to declare. Why not? The emphasis on: false swearing (perjury) and the act of telling lies (more than joining an associate with Allah) and (the act of showing ingratitude to parents) clearly stands as a sound proof of its importance in Allah's sight as it incurs upon false swearing many corruptions and serious grievances like upsetting justice and right and supporting falsehood, injustice, deceiving the judiciary and the loss of people's rights.
Then, the heading with which the Messenger (P.B.U.H.) started the hadith and his repetition of swearing falsehood would surely wear away the Islamic body of the society where the rights of people may be lost, vexation and grudges amongst people grown which in the end lead to committing crimes and disrespecting people's honors and dignities.