$a = zeroes 10,10; $$a{FOO} = "bar"
doesn't work. They are currently scalar references (to opaque C structures in finer terms) because of speed as well as syntactic issues. If you want to have a hash, use
$a->hdr()
which returns a reference to an anonymous hash. Also, subclassing works if you store a piddle in the hash member ``PDL''.
There are also many core enhancements to support Dataflow and Slicing tricks, but these do not introduce any incompatbilities.
The INSTALL file for how to build PDL.
The README file for general stuff.