Try this:

sub start {
    $tl = $mw->Toplevel;
    $tl->title("B");
    $tl->focus;
    $tl->grab;
    $tl->Button( -text => "B", -command => sub{ destroy $tl } )->pack;
}
[download]

title is documented in Tk::Wm;
focus and grab have their own doc pages.