$mainwindow->bind( 'Tk::Text', '<Leave>', [\&copyText, $tw1, $tw2] );

##</code><code>##

sub copyText {
  my ($trigger,$tw1,$tw2) = @_;
  my ($src,$dst,$txt);
  if ($trigger eq $tw1) {
    $src = $tw1;
    $dst = $tw2;
  } else {
    $src = $tw2;
    $dst = $tw1;
  }
  $txt = $src->get('1.0','end');
  $dst->delete('1.0','end');
  $dst->insert('1.0',$txt);
}