Yeah, there is a lot of stuff out there still runs on 5.005_03. I work on a large application under active development that still runs under perl 5.005_03 (although we are planning to upgrade perl this year). To quote one of my sysadmins, "for many years, Redhat 6.2 essentially was Linux," and 5.005_03 was the perl that came with RH6.2.

However, there are certainly "unsafe" things about perl 5.005_03, for example, it's non-reentrant signal handling.