If God created a perfect world (re: Genesis) then how did sin get a foot in the door?

Specifically, and from what I think I know (so some of this may be factually incorrect) but if in Genesis (re: The Bible) God created a perfect world with Adam and Eve then how exactly did sin enter into the equation to entice Eve (or was it Adam?) to do the "first wrong". If we are all to live on Earth to finally die and then go to Heaven then it worries me greatly that Heaven will not be perfect as (and here is my point) that is what I originally thought God created in the first place for us all when I read about the creation of the Universe, Heaven and Earth and Man in Genesis.