I believe that use varies from require in that it occurs at compile time, not run-time, thus, if you have a module 'Module' that can only be loaded if you're using 5.6, the following bits of code will function differently:

require 5.6;
use Module;

# Module is loaded *before* 5.6 test is done

use 5.6;
use Module;

# If we're < 5.6, we get an error message before Module is loaded.
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If I'm incorrect in this, by all means correct.