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On my most-recent project, I did something I had not properly done before: I built an ongoing test-suite, during the development process. As each new feature was added to the code, and sometimes before, I immediately wrote a test-case for it. And I didn't proceed until that test, and all preceding tests, “ran clean.”
This had immediate effects:
Fortunately for all of us, Perl makes it very easy to write tests. A test program is simply a Perl program, written using a test-module such as Test::Most, Test::Memory::Cycle, Test::Exception, or Test::Class. You see test-suites run every time you install anything from CPAN, so it's a very well-developed system. When you start writing test-suites of your own, you really appreciate what it means when you see hundreds or even thousands of test-cases flying by during one of those installs. The fact that “CPAN is highly reliable” is anything but an accident.
What I do (and it's not the only way nor necessarily the best way to do it) is simply to create a t subdirectory, for “tests,” with number-prefixed subdirectories and number-prefixed test-programs in each. (This is done because the test files and directories will be traversed in-order.) Then, I run these with the command: prove -r.
A test-case is very simple, really: it's both a list of things that you do expect to happen, and things that you don't. It's a test of things that should cause an exception to occur, and things that shouldn't. It's a test of the “edge cases,” the sublime, and the ridiculous.
For web-based applications you can go further. Modules such as Test::WWW::Mechanize can perform an entire web-interaction sequence. Or you have things like Test::WWW::Declare, which is based around the notion of defining a “flow.”
My point here is not to enumerate all of the things that you can do with Perl testing. (There are, today, 805 CPAN modules whose name begins with “Test::” ...) Rather, it is: “You should do this. It really works.” Sure, I'm not the first person to say it. I'm just one who took about twenty years to really listen. ;-) ;-) :-D