use warnings;
use strict;
use XML::LibXML;
use open qw/:std :utf8/;

my $xml = <<'END_XML';
<?xml version="1.0" encoding="ISO-8859-15"?>
<foo></foo>
END_XML

my $doc = XML::LibXML->load_xml(string => $xml) or die;
$doc->documentElement->appendText("\x{20AC}");

open my $fh, '>', '/tmp/one.xml' or die $!;
print $fh $doc->toString;
close $fh;

$doc->toFile('/tmp/two.xml');