#!/usr/bin/perl -w
use strict;

use XML::XPath;

undef $/;

my $XML=<DATA>;

my $xpath = XML::XPath->new('xml' => $XML);
foreach my $node ($xpath->findnodes('/foo/bar/text')->get_nodelist) { 
    my $data = your_munge_function($node->string_value);
    my $id = $node->getAttribute('id');
    $xpath->setNodeText('/foo/bar/text[@id="' . $id . '"]', $data);
}


my $newXML = $xpath->findnodes_as_string('/');

# safe because XML::XPath entiti-zes > in attributes
$newXML=~ s{(<text[^>]*>)(.*?)(</text>)}
           {$1 . cdata_ize($2) . $3}eg;

print $newXML;

sub your_munge_function
  { return "munged $_[0]"; }

sub cdata_ize
  { my $text= shift;
    $text=~ s{&amp;}{&}g;
    $text=~ s{&lt;}{<}g;
    return "<![CDATA[$text]]>";
  }

__DATA__
<foo>
<bar>
<text id="text1>"><![CDATA[La dee da de da.<br>Foo bar baz]]></text>
</bar>
</foo>