Here is the code which will help you to understand HOH

use Data::Dumper;
$hash{a1}{b1} = 'c1';
$hash{a1}{b2} = 'c2';
$hash{a2}{b1} = 'c3';
$hash{a2}{b3} = 'c4';

print Dumper(\%hash);


foreach $key (keys %hash){
    print "Key:\t$key\n";
    my %subHash = %{$hash{$key}};
    print "\tSubkeys:";
    foreach $subkey (keys %subHash){
    print "\t$subkey";
    }
    print "\n";
}
[download]

__END__

$VAR1 = {
          'a2' => {
                    'b1' => 'c3',
                    'b3' => 'c4'
                  },
          'a1' => {
                    'b1' => 'c1',
                    'b2' => 'c2'
                  }
        };  

Key:    a2
        Subkeys:        b1      b3
Key:    a1
        Subkeys:        b1      b2