The project is still being developed with me as the primary coder. My sense was that a new version of Perl would make me more productive and allow for a cleaner code base. I use modules by switch and say but there's no way to replicate other features such as '//='
Of course, it's hard that quantify benefits like that. It's true that there's a one time cost to upgrading and that it would be possible to continue development with 5.8. On the other hand, just because a technology works doesn't mean it's optimal to continue using it. For example, Perl 4 could also be used for a project but Perl 5 clearly results in better productivity and higher quality code.