lanx@nc10-ubuntu:~$ perl -MData::Dumper -e ' %h1=(3 => 1, 5 => 1, 10 => 3 ); %h4=%h3=%h2=reverse %h1; print Dumper \%h2,\%h3,\%h4'
$VAR1 = {
          '1' => '3',
          '3' => '10'
        };
$VAR2 = {
          '3' => '10'
        };
$VAR3 = {
          '3' => '10'
        };