I have never heard the second term used before but the great www suggests that both are variations of the latin 'annum' = year. Annum/annual - is a yearly plant ie it dies off after flowering etc. Per-annum/perennial - comes up every year. I can only think that you have seen the term used on an item of foreign origin and it has been poorly translated into English.

As Bulba says annuals grow from seed, flower and die off completely all in one year. Perennials grow for many years and can be herbaceous, meaning all the top growth dies in autumn to reappear the following spring, or the top growth may remain over the winter.