my $drag_obj = 0;

##</code><code>##

sub hover_over_drop {
    my ($label_obj, $b_entry, $x_pos) = @_;
                                                                                
    $label_obj->configure(-bg => $b_entry == 1 ? 'gray' : 'lightgray');
                                                                                
    if ($b_entry) {
        if (!$drag_obj) {
            print "First time:  ";
            $drag_obj = $label_obj;
        } else {
            print " Next time:  ";
        }
        printf "Dragging(%s)  Callback for(%s)\n",
            $drag_obj->cget(-text),
            $label_obj->cget(-text);
    }
}

##</code><code>##

sub perform_drop {
    my ($drop_site, $x_pos, $drop_obj) = @_;
                                                                                
    $drag_obj or die "Whoops -- \$drag_obj undefined (shouldn't happen!)\n";
    printf "Drop label:  %s\n", $drag_obj->cget(-text);
    $drag_obj = 0;
}

##</code><code>##

More information can be found in the DragDrop directory in the source distribution of Perl/Tk. There are some sample scripts: local_test, motion_test and site_test. And if there are still open questions: use the force, read the source!