If you want to benchmark explicitely clearing of the array, you should do that - and not start with a "new" array (although, behind the scenes, the array isn't quite new) each time. Move the declaration of the array outside of the loops. Something like:

our @working;
cmpthese -20, {
   one => '@working = (); $working[$_] = $_ foreach 0 .. 100',
   ...
};
[download]

Of course, if you're going to fill in all the elements anyway, why bother clearing them?