Please have a look at How do I post a question effectively? - you haven't given a runnable piece of code, the expected output, and the current output.

You seem to want to avoid passing the document object around like it's a bad thing - it's not, in fact, if I understand your question correctly, then the solution is based on that you will need to pass the XML::LibXML::Document object around and add your elements to the document as you create them, instead of handling unbound nodes.

use warnings;
use strict;
use XML::LibXML;
my $doc = XML::LibXML::Document->new;
my $el1 = $doc->createElementNS("namespace0","aaa");
$el1->setNamespace("namespace1","p1",0);
$el1->setNamespace("namespace2","p2",0);
$doc->setDocumentElement($el1);
my $el2 = $doc->createElement("bbb");
$el2->setNamespace("namespace2","p2",1);
$el1->appendChild($el2);
my $el3 = $doc->createElementNS("namespace0","ccc");
$el1->appendChild($el3);
my $el4 = $doc->createElement("p1:ddd");
$el3->appendChild($el4);
print $doc->toString(1);

__END__

<?xml version="1.0"?>
<aaa xmlns="namespace0" xmlns:p1="namespace1" xmlns:p2="namespace2">
  <p2:bbb/>
  <ccc>
    <p1:ddd/>
  </ccc>
</aaa>
[download]

Note how the namespace declarations are automatically reused. This is actually documented in XML::LibXML::Element's setNamespace. Since you seem to want to work with XML::LibXML more closely, I strongly suggest you take the time to read all of its documentation (starting with XML::LibXML::Node, XML::LibXML::Element and XML::LibXML::Document).

you haven't given a runnable piece of code

True. My bad. Sorry

That said I have worked out the XML::LibXML approach.

For example I want to build:

<?xml version="1.0" encoding="UTF-8"?>
<O:ABC xmlns:O="ONE:" 
       xmlns:T="two:IS:a_namEsp">
  <O:ZZZ/>
  <T:ZZZ>
    <O:QWERTY>
      <T:UIOP/>
    </O:QWERTY>
  </T:ZZZ>
</O:ABC>
[download]

The following code does it....

#!/usr/bin/perl -w
use strict;
use XML::LibXML;

# NAmespaces
my $ONE_ns = 'ONE:';
my $ONE_pfx = 'O';
my $TWO_ns = 'two:IS:a_namEsp';
my $TWO_pfx = 'T';

# The XML Document
my $DOC = XML::LibXML->createDocument( "1.0", "UTF-8" );

# Set the root
my $root = $DOC->createElement('ABC');
$DOC->setDocumentElement($root);

# Establish namespaces
$root->setNamespace($ONE_ns, $ONE_pfx, 1);
$root->setNamespace($TWO_ns, $TWO_pfx, 0);

# Build the document tree
my $this = &appendOZ($root);
$this = &appendTZ($this);

print $DOC->toString(1);

sub appendOZ {
    my $root = shift or die;
    $root->addNewChild($ONE_ns, 'ZZZ');
    return $root;
}

sub appendTZ {
    my $root = shift or die;
    my $this = $root->addNewChild($TWO_ns, 'ZZZ');
    &appendOQ($this);
    return $root;
}

sub appendOQ {
    my $root = shift or die;
    my $this = $root->addNewChild($ONE_ns, 'QWERTY');
    &appendTU($this);
    return $root;
}

sub appendTU {
    my $root = shift or die;
    $root->addNewChild($TWO_ns, 'UIOP');
    return $root;
}
[download]

Where a sub tree is a leaf it says:

    my $root = shift or die;
    $root->addNewChild(<namespace>, <name>);
    return $root;
[download]

Where the sub-tree is the root of another sub-tree itself it says:

sub appendOQ {
    my $root = shift or die;
    # Make this node
    my $this = $root->addNewChild(<namesace>, <name>);
    # Append sub tree
    &appendTU($this);
    return $root;
}
[download]

This works out nicely and does what I want. If I want a different tree:

<?xml version="1.0" encoding="UTF-8"?>
<O:ABC xmlns:O="ONE:" xmlns:T="two:IS:a_namEsp">
  <O:ZZZ/>
  <T:ZZZ>
    <O:QWERTY>
      <T:UIOP/>
    </O:QWERTY>
  </T:ZZZ>
  <O:QWERTY>
    <T:UIOP/>
  </O:QWERTY>
</O:ABC>
[download]

I change the core of the programme to:

# Build the document tree
my $this = &appendOZ($root);
$this = &appendTZ($this);
$this = &appendOQ($this);
print $DOC->toString(1);
[download]

What I expected to be writing, when adding a sub-tree, was some thing like:

    # Make this node
    my $this = $root->addNewChild(<namesace>, <name>);
    # Append sub tree
    $this->append(&buildSubTree)
    return $this;
[download]

The only reason (I can see, with my inexperience) to pass the root is for the namespace prefix information. It is nice to use prefixes, and I can see with hindsight that it would require some post processing of the tree to achieve that. So what I was expecting is not the "XML::LibXML" way. Fair enough

So the answer to my question was "No, yes". Simple really

Another thing. Some one said:

I strongly suggest you take the time to read all of its documentation

. Sensible advice. I have been doing exactly that. There are thousands and thousands of lines there, and they are organised as a reference not an introduction. It is hard to start the learning curve with XML::LibXML. In the absence of an introductory document there will be stupid sounding questions from beginners.

create ABC ;
cd ABC ;
declare-ns O ONE: ;
register-namespace O ONE: ;

declare-ns T two:IS:a_namEsp ;
register-namespace T two:IS:a_namEsp ;

set-ns ONE: O;

insert element O:ZZZ into . ;
my $t := insert element T:ZZZ into . ;
my $q := insert element O:QWERTY into $t ;
insert element T:UIOP into $q ;

insert element O:QWERTY into . ;
insert element T:UIOP into O:QWERTY ; # No need to create variables ev
+erytime.

save :f new.xml ;
[download]

It produces almost the XML you posted in a reply, the only difference is utf is lowercase in the declaration.