use warnings;
use strict;
use XML::LibXML;

my $doc = XML::LibXML->load_xml(string => <<'XML');
<foo>hello, foobar <bar x="foo">world! foo</bar> fo<quz/>oofoo</foo>
XML

my $re = qr/foo/; # don't include any capturing parens in this!

my @nodes = $doc->documentElement;
while (my $node = pop @nodes) {
	for my $c ($node->childNodes) {
		if ($c->nodeType==XML_ELEMENT_NODE) { push @nodes, $c }
		elsif ($c->nodeType==XML_TEXT_NODE || $c->nodeType==XML_CDATA_SECTION_NODE) {
			my @parts = split /($re)/, $c->data;
			next unless @parts>1;
			my $d = $doc->createDocumentFragment;
			for my $i (0..$#parts) {
				if ($i%2) {  # regex match
					my $e = $doc->createElement('x');
					$e->appendText($parts[$i]);
					$d->appendChild($e);
				}
				else {  # text around match
					$d->appendText($parts[$i]);
				}
			}
			$node->replaceChild($d, $c);
		}
	}
}

print $doc->toString;

__END__
<?xml version="1.0"?>
<foo>hello, <x>foo</x>bar <bar x="foo">world! <x>foo</x></bar> fo<quz/>oo<x>foo</x></foo>