use strict;
use warnings;
use Data::Dumper;

my %hash= (10=>1, 20=>2, 30=>1, 40=>2);
my %rhash;

map {$rhash{$hash{$_}} += $_} keys %hash;

print Dumper (\%hash);
print Dumper (\%rhash);
##</code><code>##

$VAR1 = '%hash';
$VAR2 = {
          '40' => 2,
          '30' => 1,
          '10' => 1,
          '20' => 2
        };
$VAR1 = '%rhash';
$VAR2 = {
          '1' => 40,
          '2' => 60
        };