... tighter encapsulation ...

I doubt any of this will be new to you, but just to clarify my thoughts for myself, I suppose what I really had in mind was actually two separate and disparate effects:

• If a 'private' closure is not further encapsulated in a use-d module, pesky order-of-evaluation effects can manifest:

  >perl -wMstrict -le
  "print 'before: ', defined S() ? S() : 'undefined';
   ;;
   {
     my $x = 42;
     ;;
     sub S { return $x; }
   }
   ;;
   print 'after: ', defined S() ? S() : 'undefined';
  "
  before: undefined
  after: 42
  [download]

• If such a closure is encapsulated in a used-d module, the order-of-evaluation problems go away (assuming you're not doing any fancy-schmancy CTFE in the module, I think this is reliably true), but the 'tighter' aspect, strictly speaking, then kicks in: a state variable really is private to the function in which it's defined, but 'private' closures allow the sharing of access to variables among several functions — in itself, a highly useful property in certain circumstances!

  >perl -wMstrict -le
  "print M::get_x();
   ;;
   use M;
   ;;
   print M::get_x();
  "
  42
  42
  [download]

  Where M.pm is:

  package M;
  
  { my $x = 42;
    sub get_x { return $x; }
    sub set_x { return $x = $_[0]; }
    }
  
  my $y = 1729;
  sub cannot_access_x { return $y; }
  
  1;
  [download]

Oh, and:

... consistently 2% faster ...

I'm not sure I really believe in a 2% performance difference reported by Benchmark, but as long as it's you...!