knoppix@Microknoppix:~$ perl -MData::Dumper -E '
> sub double { $rsVal = shift; $$rsVal *= 2 }
> 
> $rhData = { one => 1, two => 2 };
> say Data::Dumper->Dumpxs( [ $rhData ], [ qw{ rhData } ] );
> 
> double( \ $rhData->{ two } );
> say Data::Dumper->Dumpxs( [ $rhData ], [ qw{ rhData } ] );'
$rhData = {
            'one' => 1,
            'two' => 2
          };

$rhData = {
            'one' => 1,
            'two' => 4
          };

knoppix@Microknoppix:~$