I32 lvalue = PL_op->op_flags & OPf_MOD || LVRET;

##</code><code>##

sub foo :lvalue {
   LVALUE {
      ...
   }
   RVALUE {
      ...
   }
}

##</code><code>##

use Variable::Magic qw( wizard cast );

{
   my $wiz = wizard(
      # Called before reading the magic scalar (${$_[0]}).
      # Assign the value to ${$_[0]} that you want to be fetched.
      # $data is $_[1].
      get => sub { ... },

      # Called after an assignment to the magic scalar (${$_[0]}).
      # $data is $_[1].
      set => sub { ... },
   );

   sub foo :lvalue {
      LVALUE {
         my $data = ...;
         cast(my $magical, $wiz, $data);
         $magical
      }
      RVALUE {
         ...
      }
   }
}