How to have a drupal module respond on node creation
All credits to Clive, but here is the answer.

You need to write a custom module: Simply write a MYMODULE_node_insert($node) function containing the code you want to execute. Drupal will call that function every time a node is created (when the module is enabled).

If you are changing the code of a module that is already enabled, you should clear Drupal caches, as otherwise Drupal will not notice the module contains a new hook implementation. This can be achieved by disabling the module before editing it, and re-enabling it after the code has been changed.