my $w;
$w->{main} = MainWindow->new;
$w->{other_widgets} = $w->{main}->Label->....
# and then later...
&subroutine($w);

sub subroutine
{
    my $w = shift;
    # Do stuff with $w here now like normal
    $w->{main}->configure( -title => "new title" );
}