My employer (a brokerage) uses three machines per Production product:

1. development, where it doesn't really matter what state the software or environment is in
2. QA, where the latest stable environment / software is available for testing
3. Production.

Please keep in mind that some of us may be talking at cross-purposes. My software only runs under one version of Perl under Solaris, and is maintained solely for our use. If you were developing classes that needed to run under Perl 5.0, 5.8 and 5.10 on user-owned machines, I see the necessity of multiple Perl environments on one box.