Lets not forget about XML.

XML::Simple makes for an excellent app config also.

app.rc:

<app>
  <db>
    <username>Python</username>
    <password>sucks</password>
  </db>
  <backup>
     <machine>guido</machine>
     <frequency>4 hours</frequency>
  </backup>
  <logging>
     <file>/var/log/applog</file>
     <level>2</level>
  </logging>
</app>
[download]

Derives the same access:

$app->{'db'}->{'username'}
[download]

As well as all the good stuff that comes with xml, such as for checking the "config" file ( actually just another xml doc ), for well-formedness and validity. Also, XML will provide ties for easy config-file update and modification via the application itself, should you choose to use something like XML::Writer. A fun and nice way of getting fancy for say, a web editable/creatable configuration file.