...but I want to use XML for the config file

I hope for the sake of your immortal soul, that in your xml you store something more complicated than

<options>
  <section name="users">
    <option name="larry" value="in0n3line?" />
    <option name="dominus" value="errorme$$age!" />
  </section>
</options>
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Such data, which could far easier (and more robust than with XML::Simple (sometimes it's a PITA to get XML::Simple to parse or write how you want it to)) written as classic INI-Files. Using XML for this may be considered NTBI™ (Not The Best Idea). For a deeper discussion refer Configuration file design an my reply to it.