Is installing modules in site_perl by setting installdir=site sufficient protection
Using site_perl should be ok (though I'm not entirely sure how exactly Red Hat is packaging their stuff), but as site_perl would typically be later in the default @INC, you'd have to change that temporarily in order for your own module versions to be found (in your development environment) in case there are competing versions.
Anyhow, the common advice is to stay away from the system Perl entirely, and install your own Perl in some other directory (e.g. /usr/local/...). That way, you can mess with it any way you like without impeding the system Perl or being impeded by system upgrades. (When you build your own perl on that box, make sure to configure it to not include existing lib directories (of the system Perl) into the default @INC of the newly built perl (as is the configure default).)