Programming Perl, Second Edition, Wall, Larry, Tom Christiansen
& Randal L. Schwartz, O'Reilly & Associates, Inc. September 1996.
The Perl 5 debugger man page,
http://www.perl.org/CPAN/doc/manual/html/pod/perldebug.html.
All listings referred to in this article are available by anonymous download in the file
ftp://ftp.ssc.com/lj/listings/issue49/2484.tgz.
Also included in the tgz file is a CGI program for you to debug.