sub _xml_serialize {
    my $self = shift;
    
    my($parent_node, $tree) = @_;
    
    no warnings 'uninitialized';
    
    while(my($k, $v) = each %$tree) {
        if(ref $v eq 'HASH') {
            my $child_node = XML::LibXML::Element->new($k);
            $self->_xml_serialize($child_node, $v);
            $parent_node->appendChild($child_node)
        }
        elsif(ref $v eq 'ARRAY') {
            foreach(@$v) {
                my $child_node = XML::LibXML::Element->new($k);
                if(ref eq 'HASH' or ref eq 'ARRAY') { $self->_xml_serialize($child_node, $_) }
                else { $child_node->appendText($_ || '') }
                $parent_node->appendChild($child_node)
            }
        }
        else {
            $parent_node->appendTextChild($k, $v || '')
        }
    }
}

##</code><code>##

$self->_xml_serialize(
    $doc_root,
    outer => { inner => 'value' }
)

##</code><code>##

...<outer><inner>value</inner></outer>...