If Unicode is a vital part of your project, specifying just "5.8" is not enough. I really cannot think of any application running code to deal with Unicode reliable on anything older than 5.8.4. In the (way too) long development life-cycle of 5.8.x Unicode support has hugely improved, so 5.8.8 will do MUCH better than 5.8.5. This trend set on in subsequent releases, and if Unicode is core business, moving towards 5.14.2 is a very healthy move. The jump is big though, and you will have to test everything very carefully.

Using modules like Switch is a VERY bad idea in 5.8, especially when you plan to use defined-or in 5.10 and on. They do not combine and scripts will crash in unexpected ways. I always used perl 5.8.x with defined-or (//=) built-in ( yes, there is a patch available to make 5.8.x have // natively), so I hit that problem in early stages and decided to never use Switch ever again.

Then came given/when, but it was based on smart-match, which has its own snakepit of unpredictable issues.

Summarizing: a move from earlier 5.8 to later 5.8 is warranted by 64bit issues and a little bit of Unicode. A move to 5.10 is warranted by the defined-or being available by default and much better CPAN support: authors are dropping 5.6 by the masses and 5.8 will be dropped rather sooner than later. A move towards 5.12 or 5.14 is good if your Unicode processes are core business.