$ perl modify_category.pl
<?xml version='1.0'?>
<records>
  <record>
    <publisher>O&apos;Reilly</publisher>
    <url>http://www.oreilly.com/catalog/learnperl4/</url>
    <category>Programming,Learning,Perl</category>
    <title>Learning Perl</title></record>
  <record>
    <publisher>O&apos;Reilly</publisher>
    <url>http://www.oreilly.com/catalog/9780596529864/index.html</url>
    <category>Programming,Learning,Ruby</category>
    <title>Learning Ruby</title></record>
  <record>
    <publisher>O&apos;Reilly</publisher>
    <url>http://www.oreilly.com/catalog/lpython2/index.html</url>
    <category>Programming,Learning,Python</category>
    <title>Learning Python</title></record>
</records>