$ perl -E'
    my $buf = "";
    {
        open my $fh, ">", \$buf;
        utf8::upgrade( my $all_255_bytes = join "", map chr, 0..255 );
        say length $all_255_bytes;
        say do { use bytes; length $all_255_bytes };
        print $fh $all_255_bytes;
    }
    say length($buf);
'
256   length without use bytes
384   length with use bytes
256   actual content length