notepad++ normally recognizes the encoding correctly.

I want the output to be in utf-8. I read it at a later stage to display the text in a Text widget.

The reading works so:

open (IN, "<:utf8", "output.txt");
my $in  = <IN>;
    while ($in) {
                #doing some formatting
        $in =~ s/\'//g;
        $in =~ s/\=//g;
        $in =~ s/\>//g;        
        $in =~ s/(.*)(left)(.*)/$1$2$3/g;
        $text->insert('end', "$3");
        }
    $in  = <IN>;
    }
close IN;
[download]

Of course the text wiget doesnt show corectly the äöü.

I am not an expert of encodings, but I could cope with all other encoding issues so far...

Perhaps it only switches to UTF-8 mode automatically when the document starts with a BOM. Try adding "\x{FEFF}" to the start of your document.

@ Corion (Sage): yes, I use Text Wigdet with utf8 without problems

@ ikegami: no changes also adding your line.

I am puzzeld, as all this doesn't seem to make any sense.

Are you sure that your (Tk?) widget understands UTF8?