You might want to add the 'style' tag, I think the win32::gui wants to know what kind of combobox it is. Try the code below:

$box = $main->AddCombobox( 
    -name   => "testbox",
    -left   => 20, 
    -top    => 35,
    -width  => 600, 
    -height => 500,
    -style  => WS_VSCROLL | WS_VISIBLE | 3 | WS_NOTIFY,
            );
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I think the WS_VISIBLE will be the key to your success.