DCR submit with External Task to DD File problem

System

I have a simple workflow that has and External and an End Task and is initiated by a save DCR (tt_data). The external task calls a perl script I have written which in turn calls iwdd.ipl to datadeploy the DCR to the Staging DB (Note I am not using DAS as I have a User-defined Schema and only a single Workarea).
My problem is how do I know, and pass onto iwdd.ipl which category/type and even DCR that I want to deploy ?
Are these values not passed to the Workflow from the DCT somehow ? Is this via 'files', because when I do GetFiles in my 'wrapper' perl script I get nothing.
Must I do a Submit Task first ?
Many thanks in advance

/Nick