Fix exception when $_ is modified in before/after.

Fix exception when $_ is modified in before/after.

I _think_ what's happening here is that $_ was getting aliased to an entry in
@$before/@$after, so that modifying $_ would in fact modify that array slot's
scalar.

Declaring `my $c` like this doesn't seem to impact performance vs. using $_.
Putting it outside the "cache" closure means `my` only gets run once.