It has been a while since there has been a good round of golf, so I decided to post one. I couldn't come up with a nice interesting math brain-teaser, so I'll post this one.

The Rules
Expected Input-Output
My Swing (83 chars)
sub a { $_=pop;($_=reverse sprintf"%.2f",$_)=~s/(\d\d\d(?=\d))/$1,/g,return re +verse split'' }


(update: Changed "integer" to "number". Integers don't have decimals. Thanks to xylus for pointing out my err.