t/immutable; "fix" 5.19.3 failures

5.19.3 introduced some funky test failures:
  http://www.cpantesters.org/cpan/report/f5e5f584-0b73-11e3-98e5-f97f5735fcd0
The backing ARRAY is still successfully marked read-only (you can't push to
it, for example) but individual elements of the array are not.

If you're doing this, you're breaking encapsulation anyway, but it might be
nice to have! Revisit later.

	modified:   t/immutable.t