What arguments could be used to justify an upgrade like this?
I'd say, that if you, with knowledge of the needs of your company cannot answer that question, and have to ask a bunch of nerds without any idea of your business processes for justification, I'd say, you don't have any justification what so ever.
Let me ask you, is upgrading to 5.10 (or 5.12 or 5.14) going to make your company more money (now and in the near future)? Don't forget to deduct the cost of the upgrade itself from any increase in revenue (or to add it to whatever savings an upgrade is giving you). If you cannot make a business case for it - don't upgrade. Or at least, don't do it now. Don't forget that your customers don't give a flying fuck whether your server runs perl 5.8, perl 5.10, perl 5.14, perl 6, ruby, python or something else. They just want whatever service or produce you're providing.