Yes it exists, I just got done building an installation on a Sun box.  For the full story, please heed the advice of the previous post and read the INSTALL file that should have come with the perl source.  Some advantages are you get to address greater than 2GB of memory and the 255 max open file-handles barrier goes away.  The disadvantages these pose are if you create a module that depends on these (or other 64 bit exclusive features) people who are running 32 bit will curse you when they try using your module.