$$var is just a short form for ${$var}. the $var inside the braces is a reference; in this case a reference to a scalar.
you dereference a variable by putting ${ } around it. it $var were a reference to an array, you'd dereference it by putting an @{ } around it, and %{ } for a hash.
please read more about it in perldata, perlref etc...
if the $var is not a reference but just a simple string, then we're dealing with symbolic references here, which you should avoid...