If "upgrade" means: Install a new version from CPAN, then you should be safe. If a core module is available on CPAN, then this is exactly why: so that you can upgrade it without upgrading Perl.

The details vary between different platforms, but there are safeguards in place:

• Core modules come with a decent set of tests. Run them, as one usually does when installing from CPAN. If one of the test fails, then the module refuses to install - no harm done.
• Modules installed from CPAN end up in other directories (with higher priorities in @INC) than those coming with the core. So you don't overwrite existing files. Just keep track where the files go so that you can backpedal if desired.

Mostly, this also applies to XS modules. If you install them from CPAN, they will be compiled and tested against your installed version of Perl, and end up in a directory specific for your version, but still different from the directory where you got it with Perl. The rollback is a bit more complex because more than one directory needs to be "cleaned".