$string = "Test ä ö ü Ä Ö Ü ß";

$string =~ s/ä/ä/g;
$string =~ s/ö/ö/g;
$string =~ s/ü/ü/g;
$string =~ s/Ä/Ä/g;
$string =~ s/Ö/Ö/g;
$string =~ s/Ü/Ü/g;
$string =~ s/ß/ß/g;

print $string, "\n";