That all depends on how you installed your new perl version, and how you are installing your modules. From the sound of it, you're just installing RH rpm packages.

(Is there an rpm of 5.8.1 already?) - If you installed an rpm, then it will be pre-compiled with a default path in which it looks for modules, (see perl -V, as described above.) OTOH, if you built it yourself from source, then you should have had an opportunity to look for previous perl versions, and add their lib paths to the new 5.8.1 binary. If the 5.6.0 lib paths are not in the binary, then you'll need to set PERL5LIB.

As for modules, same story. If you are just installing module packages, they will land in whichever lib directory they were packaged as. (See rpm -ql <packagename> for the location of the files.) If you are installing modules 'by hand' from CPAN, then it depends which perl you are using to install them, install with 5.8.1, and they land in the 5.8.1 lib path.

The default for 5.6.1 should be /usr/lib/perl5, and for 5.8.1 /usr/local/lib/perl5.

C.