In the current PDL git (essentially 2.4.7) t/plplot.t fails on my Mac (10.6.4). Apparently it only fails on OS X, not on other OSs (Puneet Kishor reported this some time ago (see http://mailman.jach.hawaii.edu/pipermail/perldl/2010-August/004361.html and http://mailman.jach.hawaii.edu/pipermail/perldl/2010-September/004474.html\). I have traced the problem all the way back to c_plshades() in the ppdef('plshades') call in plplot.pd (around line 3134).
It does not seem to be a problem with the underlying plplot library, since example x16c (which demonstrates plshades) works just fine. For reference, my plplot is 5.9.6, installed using MacPorts (though I believe Puneet installed from source manually and also has the same problem). Assigning to Doug Hunt for now unless someone else volunteers. perldl -V output attached.