That's about the only easy thing you can do with a PDF (aside from unlink()). Here's how to get a page count.
my $pdf = PDF::API2->open( 'yourfile.pdf' );
my $count = $pdf->pages();
print "O frabjous day! The PDF has $count pages. Callooh! Callay!\n";
Update: Well, in my excitement, having just done some painful work with PDF's a few days ago, I jumped on an answer, not realizing that you (the OP) followed up to your own node with a mostly similar solution. ...except that yours keeps a running total of total pages across multiple PDF files. I didn't see that need in the original question. But now I'm wondering what just happened here: If you already know the answer why did you post the question to Seekers of Perl Wisdom. Is there something more you would like us to add to your solution? :)