For everything except for short throwaway scripts, or otherwise incredibly short programs, there is no good excuse, not to use a version control system (a.k.a: "revision control systems", "source control systems", or more in general as part of "software configuration management"). This is especially true nowadays given the availability of several powerful, easy to use, open-source (and as a result free-of-charge), and cross-platform, version control systems, that you should have not a lot of problems to deploy, learn and use.
For more information and the motivation behind using version control systems, see the relevant section out of the fifth part of “Perl for Perl Newbies” for more discussion about the motivation behind that, some links and a demonstration.
Some links for further discussion:
The Free Version Control Systems Appendix of Producing Open Source Software.
The Wikipedia List of revision control software.
“You Must Hate Version Control Systems” - a discussion on Dave Cross’ blog about best practices in the software development industry.