You should have been more specific when asking your question, the question title and text provided suggests that you are changing Perl version on Solaris, rather than changing Solaris versions. See How do I post a question effectively?.

Why not simply test your script? Surely this new system is going to be tested before production use? If you don't have access to the new Solaris 10 platform, follow my previous advice, install whatever version you need elsewhere in your existing system, test your script using that verson. I wouldn't consider downgrading to 5.8.0.