Thanks for the replay
I've already solved my problem. Dunno why I haven't done it earlier... The code is simple stupid again, but seems working:

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') { $self->_xml_seri
+alize($child_node, $_) }
                else { $child_node->appendText($_) }
                $parent_node->appendChild($child_node)
            }
        }
        else {
            if($k =~ s/^-//o) { $parent_node->setAttribute($k, $v) } #
+ just one line of code for handling attributes
            elsif($k =~ s/^\+//o) { $parent_node->appendText($v) } # a
+nd one for text childs
            else { $parent_node->appendTextChild($k, $v) }
        }
    }
}
[download]

Now I can pass attributes with '-' prefix:

$qiwi->_xml_create(outer => {'-outer-attr' => 'val', '+' => 'outer tex
+t'})
[download]

Output:

...<outer outer-attr="val">outer text</outer>...
[download]