#!/usr/bin/env perl

use strict;
use warnings;

my %h = (a => 1, b => 2, c => 1, d => 2, e => 1, f => 2);
my %i; $i{$h{$_}}[@{ $i{$h{$_}} }] = $_ for keys %h;
my %j = do { $_{$h{$_}}[@{$_{$h{$_}}}] = $_ for keys %h; %_ };

use Data::Dump;
dd \%h, \%i, \%j;