Dot should be perfectly happy with ten nodes... and also with forty

In order to isolate the problem change this:

  eval {
        $self->{graph}->run(format  => 'png');
        $png   = $self->{graph}->dot_output();
    };
[download]

To this:

 eval {
        $self->{graph}->run(format  => 'svg');
        $png   = $self->{graph}->dot_output();
    };
[download]

And show us the result (or at least the first and last lines)