When updating to a new version of perl, the biggest problem I've had is getting it to include the right set of existing module directories in @INC. At times I've even hacked the problem by copying certain modules from a directory that got omitted to one that was included. My final build of Perl 5.10.1 yesterday finally got it right, so it seemed good to share how.

./Configure -Dotherlibdirs=/usr/local/lib/perl5/5.8.9:/usr/local/lib/p
+erl5/site_perl/5.8.9:/usr/local/lib/perl5/5.8.8:/usr/local/lib/perl5/
+site_perl/5.8.8:/usr/local/lib/perl5/5.8.7:/usr/local/lib/perl5/site_
+perl/5.8.7:/usr/local/lib/perl5/vendor_perl/5.8.7
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Obviously, you may want more or less entries, depending on how far back your perl module libraries go. It's also good to check if all the directories in your previous perl's @INC actually exist and contain at least one module.

For directories listed in -Dotherlibdirs, both the directory named and its "platform" subdirectory (like i386-freebsd) get included in @INC. But for responses to Configure's question about "look in other dirs", which for me offered a default of [5.8.9 5.8.8 5.8.7], only the site_perl directory seemed to be included. So answer that question with 'none', because you've already included all the directories you want in -Dotherlibdirs.

UPDATE 9/2/09: A smart fellow on the mod_perl mailing list pointed out that XS modules (in a "platform" subdirectory like i386-whatever) aren't guaranteed to be transferable to a higher major release, e.g., from a 5.8.x perl to a 5.10.x perl. One should re-compile them, if necessary with a force install xyz::abc in CPAN.

UPDATE 9/6/09: Carrying libraries across major Perl versions (like 5.8.9 to 5.10.1) is a bad idea: don't do it. When I rebuilt perl 5.10.1 without the 5.8.x libraries, Dynaloader error messages that I got trying to load my Apache server with mod_perl2 went away, and all was well.

The technique described above is still useful for upgrading perl within a major level (like 5.10.1 to 5.10.2).