If you've enjoyed doing ETL using a graphical dataflow environment, you haven't done large or complex data transformations. In my experience, these data graphs always become too large to print and document. They are also hard to diff against each other.
The Perl module coming closest to ETL is Workflow, and it wouldn't be too hard to create the appropriate "transformation" nodes to it. You only need to render it to SVG, and allow the user to modify it. Writing such a node editor shouldn't be too hard either, as there is existing knowledge embodied in Inkscape.