What is the largest system|code|site you have (>=90% of the work was your own) supported|created|inherited which was primarily in Perl?
I wrote a 10'000 lines network test utility (database with hosts, pinging, host/IP resolving, tracerouting and proxy check) with a HTML interface. At the moment I'm writing a library of useful classes for system administration scripts which has about 5'000 lines.
What are the main strategies|techniques|modules|brands-of-coffee that allowed you to pull it off (how about a bulleted list)?
Planning: Flow charts for scripts and UML for bigger libraries. Bulleted lists and TODO/FEATURES lists also help a lot. Also look at CPAN if someone has already done something similar.
Modules: CGI, HTML::Template, CGI::Application, DBI
What was the single biggest mistake (most costly to your own time) you made while doing it?
Improper design at the beginning -> rewrinting some parts from scratch and having done very little documentation (that was before I discovered POD :)).