This question comes up regularly on Perlmonks. In fact, back when I was a novice Monkling, I actually asked (a variant of) it myself *blush* at (s)printf and rounding woes.

It also, of course, regularly comes up on forums for other languages, since such phenomena are not restricted to perl.

For the best explanation I have yet to find, please read this: Re: Re: Re: Bug? 1+1 != 2, by BrowserUK