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.