Perpetual bonds are the bonds that do not have a maturity date. Most of the perpetual bonds have a call option embedded in them at which the issuer can redeem the bonds.
Risk involved with early redemption or return of the principal amount on a fixed income security. The prepayment may come at a time when the opportunities to deploy the cash are at lesser interest rates and hence it is called a risk.
A bond that gives the investors the right but not the obligation to ask for a prepayment of principal amount of a bond on a pre-defined date before the maturity date of the bond is known as putable bond.