use Modern::Perl '2015';

my $encoding = $^O eq 'MSWin32' ? 'cp1252' : 'utf8';
say "Encoding: $encoding <";

binmode(STDOUT, ":encoding($encoding)" );
binmode(STDERR, ":encoding($encoding)" );
binmode(STDIN,  ":encoding($encoding)" );

# each open() automatically uses :encoding($encoding)
use open ':encoding($encoding)';

my $res_file = "Erg.txt";
open(my $fh, ">:encoding($encoding)", $res_file)
        or die "Could not open file '$res_file'";

say $fh "Umlaute: äöüßÄÖÜ";
say $fh "Euro €";

say "Umlaute: äöüßÄÖÜ";
say "Euro €";

close $fh;