A "perpetuity" is a theoretical bond which makes payment during its entire life. One realizable example of this is a company issued stock which always pays dividends each year. The lifetime is defined from the time of issuance until buy back or potential bankruptcy.
A perpetuity, such as preferred stock, can be valued by simply dividing the payment by the applicable discount rate.
XYZ Corp. preferred stock pays a $2 dividend every year. This dividend is expected to remain constant for the forseeable future. If investors are requiring a 10% return, what is the stock selling for?
Therefore, XYZ Co. stock should sell for $20 per share.