use strict;
use warnings;
use Encode qw/encode_utf8/;
use charnames qw/:full/;
use 5.010;  # nur fuer say() benoetigt

my $chars = "\N{LATIN SMALL LETTER A WITH DIAERESIS}\N{LATIN SMALL LETTER O WITH DIAERESIS}";
my $bytes = encode_utf8 $chars;

{
    use utf8;
    say 'With utf8';
    say encode_utf8(uc($chars)), " ",  uc($bytes);
    say encode_utf8(substr($chars, 0, 2)), " ",  substr($bytes, 0, 2);
}

{
    no utf8;
    say 'Without utf8';
    say encode_utf8(uc($chars)), " ",  uc($bytes);
    say encode_utf8(substr($chars, 0, 2)), " ",  substr($bytes, 0, 2);
}
__END__
With utf8
ÄÖ äö
äö ä
Without utf8
ÄÖ äö
äö ä