Is there sin involved in signing papers for another person with the intention of helping him reclaim his right, and is that considered perjury, and does that entail legal accountability?

What you did is falsification, which is forbidden by Islamic law and is among the greatest major sins. If a person has a right, let him seek it through legitimate means. It is not permissible for you to sign and bear witness to something you did not see or hear. You must repent to Allah and rectify the corruption caused by your signature, if indeed it led to corruption. Consult specialists regarding the legal accountability.