Is any one of these preferable?

By all means, do whatever you find most preferable. You're likely the one who's going to deal with the code most of the time (and even then, you're likely won't change anything other than the version number itself). And anyone else isn't going to get confused when seeing a line that assigns a number to a variable called VERSION. It's obvious what it's for, regardless of the syntax you're using.

One point though, $main::VERSION sets the variable in the package main. I presume you're using main just as an example - normally you want to use the name of the package instead of main. Otherwise, Exporter cannot do its job.

I prefer the our $VERSION = "..."; style, but I can happily work with the other styles. I would use use vars '$VERSION'; $VERSION = "..."; if I had to create a new module that needs to be able to run on 5.005, but it has been many years since I created a module with such a restriction, and I doubt I will write another such module in the future.