I did a simple with your code, and apart from the redundant $pdf->TargetFile($_);
it seems to correctly extract the requested PDF info from my small sample of files. I noticed that PDF::Core
(which I've rarely used) has some uninitialized variables that could be cleaned up, but I also suspect that as it is a bit old now it simply doesn't grok the particular PDF you're feeding it in your test. Perhaps you should see which PDF is being scanned when that error appears, and test some other (i.e. not generated at your shop) PDFs to see if it is a version problem.
You might also look into PDF::API2 which I have found to be pretty good (and newer), though slower than PDF::Parse for header information.
I'd like to be able to assign to an luser