I write a lot of sysadmin scripts. Generally, I try to keep the scripts themselves as short as possible, with most of the work being done in modules. This has a number of advantages:

• The modules are documented with POD and have tests that exercise them in isolation.
• When writing a suite of scripts, these scripts share the module code (avoids cut n' paste coding).
• The scripts themselves are much shorter and therefore easier to understand and maintain.
• It's quicker to write new scripts because you already have a toolbox of handy routines.
• This approach scales better, especially when writing many large scripts, maintained by many different people.

Chapter 9 of the excellent book Perl Testing: A Developer's Notebook contains an interesting tactical trick to better structure scripts for testability. Their idea is to have a one-line script mainline that calls a main() function; that way you can test the script more easily from outside by hooking into the script's main() function. If you are seriously interested in Perl testing, I strongly recommend getting this book.

See also How a script becomes a module.