A few years late, but I'll record my findings here as I could not find the complete answer anywhere else (and trial and error has been kind to me).

First, to get a DropSite to accept drops, you have to provide a list of -droptypes for it to accept. This will also cause Perl to load the Tk::DragDrop::*Site/Drop modules as needed.

$sun->DropSite(
  -droptypes => ['KDE','XDND','Sun'],
  ...
);
[download]

To get the data from the drop, use SelectionGet and set the selection type. The selection type is passed as the first argument to the -dropcommand.

$sun->DropSite(
  -droptypes => ['KDE','XDND','Sun'],
  -dropcommand => sub {
    $data = $sun->SelectionGet(-selection => $_[0]);
  }
}
[download]