use Tk;
use lib ...
use App::Composite;

##</code><code>##


...
use base qw/Tk::Toplevel/;

Construct Tk::Widget 'Composite'
... implementation ...

##</code><code>##

$mw = new MainWindow;
$comp = $mw->Composite;
$comp->resizable(0, 0);
$comp->raise($mw);