Things I always do wrong

Perl really is a nice language, but it's also finicky (as I guess all computer things are these days). The mistakes here represent 90% of all the bugs in my Perl programs:

Missing semicolons: Every line outside of conditional-checking stuff needs one, and that can be a lot of lines. I usually leave at least one out. If you get an uninformative error message check to make sure that the line which caused the error and the one before it both have semicolons.
chmod +x blah.pl: Like I said in my Unix basics refresher course, you need to tell Unix which files are executables. I forget to do this about half the time. The error message you get for this and the one you get if your first line -- #!/usr/local/bin/perl -- is wrong are identical so if you get that message, check both.
Content-type: text/html: If you leave this out, things will look pretty much fine when you run the script from the Unix command line, but when you try to run it from a Web browser, you'll either get a 500 Server Error: malformed header from script, or you'll get something asking you what you want to do with a file in some format like X-URL-Encoded or something.
'=' instead of '==' or 'eq' inside conditionals: Like I said, a single = always sets the variable before it to the stuff after it. This means the line if ($name="charlie")... will be true and set $name to "charlie" unless for some reason $name can't be set (in which case it will return false).
'@' instead of '$' when referring to individual array elements: Single variables (known as scalars) always have their names prepended by $ and not by @. @names[0] can return unexpected results.

Debugging

Here are some very basic debugging tips:

Run your script from the command line (e.g. type "./foo.pl"). This will help figure out basic things like missing semicolons from the error information which Perl will return. When scripts die while being executed from a web browser, you won't get information any more useful than that it died.
Make sure the output starts with the appropriate HTTP header information. Usually this means Content-type: text/html and a blank line.
To test form information from the command line, put in a temporary line at the beginning of your script to give the script fake form information. For example:
```
          $buffer="name=jane+doe&zip=10003";
```

One last tip: You can get some fairly sophisticaed multi-part interaction by using the form element <input type="hidden">. You can record a user's previous actions and accumulate them as they go through a series of interactions. For example, you could have them walk north, east, west, or south 8 times (each time adding another hidden element to the form which they submit to determine a direction) and if at the end, they're in a specific place, they could get to some secret web area.

That's it

You've finished Jamie's crash course in CGI Programming with Perl. You should now be able to get started in the cool ;-) possibilities of interaction design on the Web.

I hope you learn to like Perl as much as I do. Happy coding.

Introduction to CGI Programming with Perl