A software like Perl, when released, should work on all major supported platforms, with all common compilers and libraries; also, with the appropriate combination of careful design decisions and luck, it should continue to work with future versions of those compilers and librairies. But it's impossible to guess what will be the future-backward-compatibility problems. For example, the recent C preprocessor for gcc 3.1 introduced a new warning (which apparently can't be turned off) that confused Perl's Configure script under some conditions. Consequently, perl couldn't be built, unless the cryptic option -Dd_attribut was passed to Configure. (if you try to build perl 5.6 with gcc 3.1, you may have to use this setting.) Funny story heh ? Conclusion : once the release candidate is officially out, test, test, download the CPAN, and test.