Thank you a lot for your ideas. At the end I took the solution of graff. It is working fine. Here my code: 
        my $unit;
        my $lesson;
        my $next;  # defaults to "undef"
        
        $available_data{'02'}{'03'} = undef;        
        $available_data{'02'}{'05'}    = undef;    
        $available_data{'02'}{'11'} = undef;

        $available_data{'05'}{'02'} = undef;
        $available_data{'05'}{'03'} = undef;
        
        $available_data{'07'}{'01'} = undef;
        $available_data{'07'}{'04'} = undef;
        $available_data{'07'}{'07'} = undef;
        $available_data{'07'}{'08'} = undef;

        for $unit ( sort { $b cmp $a } keys %available_data )
        {
            for $lesson ( sort { $b cmp $a } keys %{$available_data{$u
+nit}} )
            {
                $available_data{$unit}{$lesson}{'next'} = $next;
                $next = "$unit-$lesson";
            }
        }       

        # Unit 05, Lesson 03 --> Unit 07, Lesson 01
        $unit = "05";
        $lesson = "03";
        print "Unit " . $unit . ", Lesson " . $lesson .
            " --> Unit " . substr($available_data{$unit}{$lesson}{'nex
+t'}, 0, 2) .
            ", Lesson " . substr($available_data{$unit}{$lesson}{'next
+'}, 3, 2) . "\n";

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In reply to Re^2: Navigating in hash of hash by Dirk80
in thread Navigating in hash of hash by Dirk80

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