How do i "actually" encode the doc in UTF-8?

That depends on how you generate the document. If you create it by hand, it could look something like

use charnames ':full';

{
    my %escapes = (
        '&' => '&',
        '<' => '&lt;',   '>' => '&gt;',
        '"' => '&quot;', "'" => '&apos;',
    );

    sub xml_text
        { (my $s = $_[0]) =~ s/([<&])/$escapes{$1}/g;  $s       }
    sub xml_att_val
        { (my $s = $_[0]) =~ s/([<&"])/$escapes{$1}/g; qq{"$s"} }
}

my $s_7bit  = "A";
my $s_8bit  = "\N{LATIN CAPITAL LETTER A WITH CIRCUMFLEX}";
my $s_32bit = "\N{BLACK SPADE SUIT}";
[download]

open(my $fh, '>:encoding(UTF-8)', $qfn)
    or die;

print($fh qq{<?xml version="1.0" encoding="UTF-8"?>\n});
print($fh qq{<foo>\n});
print($fh qq{  <bar>}, xml_text($s_7bit), qq{</bar>\n});
print($fh qq{  <bar>}, xml_text($s_8bit), qq{</bar>\n});
print($fh qq{  <bar>}, xml_text($s_32bit), qq{</bar>\n});
print($fh qq{</foo>\n});
[download]

The :encoding PerlIO layer will encode the characters as UTF-8. Without the :encoding layer, the IO system will assume the characters are already encoded (and will freak out if you pass it characters that aren't bytes).

Update: Fixed bug in code (wasn't using the handle I opened). Added the required prefix to the original code.