I don't see what you've described ("Once a system was delivered to customers, upgrading Perl was no longer a possiblity
") as a basis for sticking to 5.005 (per OP, or 5.008 per current parent node?) in house.
If the customer is paying for support on a system designed for an obsolte version; just give 'em notice; your firm will no longer support any version below (pick anything 5.10 or above would be my preference).
If customer is NOT paying for support, TS on them. What are they gonna' do? Demand a refund on 13 year-old software?