At Yahoo, we had a wrapper so we could have 5.6 and 5.8 installed (and later 5.8 and 5.10), and if you had use 5.6, you'd get 5.6, and use 5.8, you'd get 5.8, and neither, you'd get the default set for that machine.
If you tried to run a 5.8 script on a machine without 5.8, you'd get an error as should be expected. If you ran a use 5.6 script on a 5.8/5.10 machine, I think you'd get 5.8, but I can't remember... 5.10 was less disruptive than 5.8.
If you tried to run a 5.8 script on a machine without 5.8, you'd get an error as should be expected. If you ran a use 5.6 script on a 5.8/5.10 machine, I think you'd get 5.8, but I can't remember... 5.10 was less disruptive than 5.8.