Instead of doing the eval 99999 times, how about

   my $per_thread_closure_maker = eval <<'   CLOSURE_HACK';
      sub {
         my ($warns) = @_;
         return sub { push @$warns, @_ };
      };
   CLOSURE_HACK

   for my $j (1..99999) {
      my @warns;
      { local $^W = 1;
        local $SIG{"__WARN__"} = &$per_thread_closure_maker(\@warns);
      }
   }
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