Update from the future:

PDL now has better, working dataflow that you can use better, with the inplace-like operation flowing. It replaces doflow. See the latest PDL's PDL::Dataflow and also Example of PDL dataflow to implement 3D space calculations.

EDIT: Having read a bit more about "dataflow programming", that's not entirely the same thing I was thinking of. PDL can (at this writing) have automatically-updated ndarrays that depend on other ndarrays. "Dataflow programming" appears to be more stream-orientated, where operations update when all their (smaller, more granular) inputs become valid. PDL can do that a little bit (see the molecule/graph-theory bit in demo 3d), but being more event-driven isn't fantastically well-supported yet.