Hi, given that the customer wants to also upgrade the OS I recommend doing that first on a test machine. Then see whether the application works as-is on the system Perl that is installed with the RedHat version you use. Next, install Perlbrew and use it to either try with the Perl version you want to use, or fall back to 5.8 and work from there.

As others have said, if you do not have a comprehensive test suite that can prove your application has not broken (as far as you know), you should develop that first before attempting any upgrades to anything.

Hope this helps!