The current PDL::IO::HDF supports version 4 of the HDF standard.
HDF5 is the standard now. Perhaps it makes sense to move to
HDF5 for PDL as well.
I notice that John Cerney has a PDL::IO::HDF5 module out.
A lot of the motivation for this would be from knowing
how much use of HDF4 or HDF5 is being made and
whether the difference matters for PDL users (current