fix failure under perl 5.26 Ever since perls lvalue subs were introduced, perl itself has been checking the OPf_MOD flag on the entersub op, not the OPf_REF flag, which was ignored. I needed an extra flag on entersub to fix a bug (the private flags are full), so I thought the OPf_REF flag was free game. I do think it is appropriate for this module to check the same flag for lvalue context that perl itself has been checking all this time.
