This seems to work for me:

use strict;
use XML::LibXML;

my $xml =<<'EOM';
<?xml version="1.0" encoding="ISO-8859-1" ?>
<form>
    <method>POST</method>
    <action>/cgi/process.cgi</action>
    <input>
        <type>text</type>
        <name>name1</name>
        <required>0</required>
        <value></value>
        <comment>This field is not required.</comment>
    </input>
    <input>
        <type>text</type>
        <name>name3</name>
        <required>1</required>
        <value></value>
        <comment>Put three words here!</comment>
        <test>
            my @tmp = split(/\ /, shift());
            return 1 if ( $#tmp == 2 );
            return 0;
        </test>
    </input>
</form>
EOM

my $parser  = XML::LibXML->new();
my $dom     = $parser->parse_string( $xml );
my $info    = $dom->createElement( 'info' );
my $failure = $dom->createElement( 'failure' );
my $reason  = $dom->createTextNode( 'blah' );

$failure->appendChild( $reason );
$info->appendChild( $failure );

my $form    = $dom->getDocumentElement;

$form->appendChild( $info );

print $dom->toString;
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