do you think anyone will cast a serious vote for less than 5.8?

Isn't it the point of such polls to determine things like this? I think that's a very valid and potentially informative option.

If I target 5.8 but increase the version any time I *need* a feature from a later perl, am I "Dynamic" or "5.8" or "Other"?

If you target 5.8 then the answer is 5.8.