To keep chromatic happy: test, test, test.

Give the code to your peers to review the code, do some QA testing.

After that, give it to a group of users that have no idea what the script is supposed to do - monitor how they're using the script - users have this annoying habit of breaking a piece of code you previously though was near prefect.

Modularise the code as much as possible and test each piece separately and as a unit.

Use CPAN modules for common tasks - they've been well tested and proven in real life, that hand-rolled parser (or whatever) hasn't.

I've already got several nodes on this topic listed on my homenode, but I'll reproduce them here:

• (OT) Security Rant
• Stay aware of security
• CGI Error Handling
• Think beyond Taint and warnings

Use Super search - I'm sure there are many more nodes on the subject.

Test, test, test.