How to add child element to a node in XMl using XML:Perl

You were on the right track with your first attempt, but you need to use insert_new_element. insert replaces the content of an element, and will create attributes with the syntax you chose.

You can put it all in a very simple twig_handler. Grab all the <version> elements, checkt their <number>'s text node and then add a new element with a text node inside for both the patch and patchv values.

The last_child => '' part puts the new element at the end of the parent. It is the $optional_position that is documented with paste.

use strict;
use warnings;
use XML::Twig;

my $twig = XML::Twig->new(
    pretty_print  => 'indented_a',
    twig_handlers => {
        'version' => sub {
            if ( $_->first_child('number')->text() eq '6.0' ) {
                $_->insert_new_elt( last_child => 'patch' )->set_text('patch');
                $_->insert_new_elt( last_child => 'patchv' )->set_text('patchv');
            }
        }
    }
)->parse( \*DATA );

$twig->print;

Output:

<Install>
  <version>
    <number>6.0</number>
    <build>1037124</build>
    <path>path</path>
    <kind>kind</kind>
    <patch>patch</patch>
    <patchv>patchv</patchv>
  </version>
  <version>
    <number>7.0</number>
    <build>1037124</build>
    <path>path</path>
    <kind>kind</kind>
  </version>
</Install>

Your second attempt does not work for at least two reasons:

# this elem get's pasted (1)
# |               on this positon (2)                            
# |               |                                     in that elem (3)
$new_elt->paste(  $number->parent->last_child->paste => $install );

In (2) it should have the position, which is a string. That's something like first_child, last_child and so on. They are listed in the docs of paste.

So it would compute the return value of $number->parent->last_child->paste and use that as the position string. But there is no such return value, so that will not work.

But it doesn't get that far. The error message cannot paste an element that belongs to a tree comes from the paste you have in (2), because you are calling paste() on $number->parent->last_child and that is already in a tree. That's not allowed. If you remove that part the code compiles.

if ( $number->trimmed_text eq $version ) {
  $new_elt->paste( $install );
}

The output is (snipped):

<Install>
  <patch>
    <build></build>
    <version></version>
  </patch>
  <version>
    <number>6.0</number>
    <build>1037124</build>
    <path>path</path>
    <kind>kind</kind>
  </version>

That is still not what we want because <patch> ended up inside <Install>, not inside <version>. But we already have $number from the loop. That is the <number> node inside of the <version> node, so we can get its parent and use that in our (3).

$new_elt->paste( $number->parent );

Now we get this:

<Install>
  <version>
    <patch>
      <build></build>
      <version></version>
    </patch>
    <number>6.0</number>
    <build>1037124</build>
    <path>path</path>
    <kind>kind</kind>
  </version>

That's better. It's kind of in the right place. If you add the position last_child it ends up at the end.

$new_elt->paste( last_child => $number->parent );

Now it's here:

<Install>
  <version>
    <number>6.0</number>
    <build>1037124</build>
    <path>path</path>
    <kind>kind</kind>
    <patch>
      <build></build>
      <version></version>
    </patch>
  </version>

Now it just is the wrong stuff we are pasting, because we wanted two elements <patch> and <patchv>, both with text. That's because you have those in $myXML.

You cannot have two top-level elements there. That's invalid XML syntax.

my $myXML = <<"XML";
<patch>patch</patch>
<patchv>patchv</patchv>
XML

That will fail with junk after document element at line 2, column 0, byte 21 at .... So you would have to either have two variables and call paste twice, or wrap them and fetch the child-nodes of the new root element.

But of course that is very complicated. The first approach above is way cleaner.