use XML::LibXML;
my $str = '<?xml version="1.0" encoding="utf8"?>'
    . "<flower>r\x{16f}\x{17e}e</flower>";
my $doc = XML::LibXML->new->parse_string($str);
warn "Document ", $doc->toString, "\n";
warn "Encoding ", $doc->encoding, "\n\n";

$doc->setEncoding('iso-8859-1');
warn "Document ", $doc->toString, "\n";
warn "Encoding ", $doc->encoding, "\n";