Oh, I see. I was looking at the documentation for version 0.1:
http://search.cpan.org/~micb/ThreadPool-0.1/Pool.pm#SYNOPSIS
which was the first hit that came on Google. I now see that I should be using the doc for version 0.3:
http://search.cpan.org/~elizabeth/Thread-Pool-0.33/lib/Thread/Pool.pm#SYNOPSIS