Self update: I've just seen App::perlimports mentioned on reddit, this might help. It seems to not actually detect unused modules by itself, but it adds a list of imports. So for example I found a script which has

use Math::Trig;

in it, but it doesn't actually use that, and App::perlimports changed that to

use Math::Trig ();

so I could tell it wasn't being used.