Beside the fact that it has "not much" to do with Perl, your question is also too general (versions of PHP, Drupal and mySQL? what hardware? were certain patches applied? what is "relatively complex"? etc. etc.). The memory consumption depends on how the application is coded and how it is used. In my limited experience with PHP and mySQL they are not memory hungry at all. My experience with Drupal is less positive and I don't use it anymore. See measuring-memory-consumption-by-drupal-bootstrap-and-modules for ideas. I think you should post your question in a PHP/Drupal/mySQL forum, they can help you better.

Cheers

Harry