#!/usr/bin/perl -w

use strict;
use Data::Dumper qw(Dumper);

my %hash = (
  a => 1,
  b => 1,
  c => 1,
  d => 2,
  e => 2,
  f => 2,
);

my %by_value = reverse %hash;

print Dumper(\%by_value);

##</code><code>##

$VAR1 = {
          '1' => 'a',
          '2' => 'e'
        };

##</code><code>##

my %hash = (
  a => 1,
  b => 1,
  c => 1,
  d => 2,
  e => 2,
  f => 2,
);

my %by_value;

while(my($key, $value) = each %hash) {
  push @{ $by_value{$value} }, $key;
}

print Dumper(\%by_value);

##</code><code>##

$VAR1 = {
          '1' => [
                   'a',
                   'b',
                   'c'
                 ],
          '2' => [
                   'e',
                   'f',
                   'd'
                 ]
        };