On July 1, 1999, and thereafter, the monthly benefit payable to the retirants and the beneficiaries of retirants of the Arkansas Public Employees' Retirement System who retired before July 1, 1989, shall be increased by an amount equal to one percent (1%) of the benefit payable on June 1, 1999, for each year or portion of a year of retirement prior to July 1, 1989.