Update: Actually, I've found the way to achieve the effect I wanted, so if anyone's interrested, here's the code you have to add just before the call to Transparent:

     $CD_map->MatteFloodfill(
          x => $x_inside, 
          y => $y_inside,
          matte => 90);
[download]

Where $x_inside and $y_inside are the coordinates of a point inside the polygone you want to make translucid. the value of matte is set to 90 here, but it seems to be more or less the same than 50% transparency...

Anyway, it's a real good way to dynamically paint area in a graphic file, once the hassle of compiling Image::Magick for Win32 is finished!