One reason to update would certainly be the ability to write better (as in more professional) code.
In perl5.6+ you have e.g. Attributes, which are a big aid in writing professional code (I use them for making abstract methods and such).
An other issue would be, that you can throw error objects

die MyPackage::Exception::DeadByDawn->new

This makes clean programming practices possible.
And I guess a lot more stuff can be thought off ;-)

But I guess this is only valid if you are still evolving your coding style, to get it more aligned with modern development practices. If you are happy with how you do it and you produce a stable code base then don't bother

Johan