lanx@nc10-ubuntu:~$ perl -MData::Dumper -e ' %h1=(3 => 1, 5 => 1, 10 => 3 ); @l=%h2=reverse %h1; print Dumper \@l,\%h2'
$VAR1 = [
          3,
          '10',
          3,
          '10'
        ];
$VAR2 = {
          '1' => '3',
          '3' => '10'
        };

lanx@nc10-ubuntu:~$ perl -version

This is perl, v5.10.0 built for i486-linux-gnu-thread-multi