Perl's debugger is powerful but somewhat esoteric. The core perldebtut tutorial is a good introduction, but who wants to skim through pages of documentation when you only need a quick refresher on the syntax of a command?
Editor's note: Unix users, we've had reports that Adobe Acrobat 5 has trouble opening this file. We're looking into this. In the meantime, xpdf and other tools appear to work correctly.