Thanks for the explanation, mirod.

I'm typically creating XML that dead-ends into a typesetting system (and may need its structure changed on a whim); therefore, DTDs are not created, and, as a result, my knowledge of them remains scant.

For my fellow SOPW, here's what I ended up with (I changed the content of element so the example would make a little more sense):

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

my $XML = XML::Twig->new(
    pi => 'process',
    pretty_print => 'indented',
    twig_roots => {
        'element' => sub {
            for my $child ($_->cut_children()) {
                if ($child->is_pcdata()) {
                    for my $piece (split /(Warning:)/i, $child->trimme
+d_text()) {
                        my $pcdata = XML::Twig::Elt->new('#PCDATA' => 
+$piece);
                        if ($piece =~ /Warning:/i) {
                            my $b_start = XML::Twig::Elt->new('#PI');
                            $b_start->set_target('xpp');
                            $b_start->set_data('bold');

                            my $b_end = XML::Twig::Elt->new('#PI');
                            $b_end->set_target('xpp');
                            $b_end->set_data('/bold');

                            $b_start->paste('last_child', $_);
                            $pcdata->paste('last_child', $_);
                            $b_end->paste('last_child', $_);
                        }
                        else {
                            $pcdata->paste('last_child', $_);
                        }
                    }
                }
                else {
                    $child->paste('last_child', $_);
                }
            }
            $_->flush();
        }
    },
);
$XML->parse(*DATA);
print "\n";

__DATA__
<root>
    <element>A sentence about the product<?xpp qa?>Warning: This may s
+pontaneously combust.</element>
</root>
[download]

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