use Encode;
use Tk;

my $bytes = "öäü";
my $text = decode("utf-8", $bytes);
 my $mw = MainWindow->new;
 $mw->Label(-text => encode("iso-8859-1", $text))->pack;
MainLoop;

##</code><code>##

 $mw->Label(-text => encode("iso-8859-1", $text))->pack;
 $mw->Label(-text => $text)->pack;