One issue will be that any XS modules will also need to be reinstalled, either using PPM, or the more traditional way. Perl 5.10 was not byte-compatible with 5.8.x.

That's probably the most significant issue; you will need to reinstall, for example, DBI, DBD::whatever, List::MoreUtils, and any other non-core modules that you might be using that happen to rely on XS code.

Of course you really should simulate your environment on another machine (or at least a virtual machine), install 5.8.9 and all the modules you depend on, then install 5.10, reinstall the non-core modules, and test, test, test. Only then ought you to upgrade production.

You may also want to check the CPAN test matrix for any non-core modules you use to verify that they continued passing their test suite on both 5.8.9 and 5.10. By now (many years later) any of the few issues there may have been have probably been worked out, but it's worth checking. And check the perldelta for 5.10, paying particular attention to Incompatible Changes.