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my $drag_obj = 0;

sub hover_over_drop {
    my ($label_obj, $b_entry, $x_pos) = @_;
...
            $label_obj->cget(-text);
    }
}

sub perform_drop {
    my ($drop_site, $x_pos, $drop_obj) = @_;
...
    printf "Drop label:  %s\n", $drag_obj->cget(-text);
    $drag_obj = 0;
}

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