$hash{'a', 'b'}

##</code><code>##

use strict;
use Tie::Hash;
package Tie::HashNKeys;
use base 'Tie::StdHash';

sub TIEHASH {
my $class = shift;
bless {}, $class;
}

sub STORE {
 $_[0]->{ join $;, sort split /$;/, $_[1] } = $_[2];
}

sub FETCH {
$_[0]->{ join $;, sort split /$;/, $_[1] };
}

package main;
use Data::Dumper;

$; = '~'; # so it can be seen
tie my %hash, 'Tie::HashNKeys';
$hash{'csUsers','csGroups'} = 'foo';
print $hash{'csGroups','csUsers'};   ## prints 'foo'
print Dumper(\%hash);