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->trimmed_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 spontaneously combust.</element>
</root>