I ran into this a while back too. Here is how I handled it:

#!/perl/bin/perl -w

use Tk;
use Tk::DropSite;
use strict;
use vars qw($top $drop @files $start_drop @null $lbox $btn );
$start_drop = time;

$top = new MainWindow;
$top->Label(-text => "The drop area:")->pack;
$drop = $top->Scrolled('Listbox',
               -scrollbars => "osoe",-height, 3, -width, 30,
              )->pack(-fill, 'both', -expand, '1');

# Tell Tk that $drop should accept drops.
# The allowed drag and drop types on Unix systems are "KDE", "XDND" an
+d "SUN"
# and on Windows systems the "Win32" dnd type.
# When dropping occurs, execute the accept_drop callback.
$drop->DropSite
  (-dropcommand => [\&accept_drop, $drop],
   -droptypes => ($^O eq 'MSWin32' ? 'Win32' : ['KDE', 'XDND', 'Sun'])
  );

$lbox = $top->Scrolled('Listbox',
               -scrollbars => "osoe",
    -height, 3, -width, 30,
              )->pack(-fill, 'both', -expand, '1');
$btn = $top->Button(-text, 'Show Files', -command, \&show_files)->pack
+();


MainLoop;

sub accept_drop {
    my($widget, $selection) = @_;

    my $filename;
    my $time = time-$start_drop;

    eval {
    if ($^O eq 'MSWin32') {
        $filename = $widget->SelectionGet(-selection => $selection,
                          'STRING');
    } else {
        $filename = $widget->SelectionGet(-selection => $selection,
                          'FILE_NAME');
    }
    };
    
    if($time > 1){ @files= @null; }
    $drop->delete('0', 'end');
    push @files, $filename;

    my $x = 0;
    foreach(@files){
    $drop->insert($x, $_);
    $x++;
    }

    $start_drop = time;
       
}

sub show_files {
    $lbox->delete('0','end');

    foreach(@files){ $lbox->insert('end', $_); }


}
[download]

I just compare the time... this will fail if someone is faster than 1 second or if the callbacks take longer than 1 second to complete... I tested it with 100 files. This is the only hack *cough, *cough... I could think of.
Hope it helps...
JamesNC
fixed typo