Turning off warnings without turning them back on is a pretty-darn-bad practice.

I'd say that any manual "turning warnings back on" is very bad practise. Even if you don't use use warnings (which has been available for more than 5 years), turning off warnings should be done by using a localized value for $^W, so the old value of $^W will be restored - no matter what the old value was, and no matter how the block is left.

  is( $^W, 1, 'warnings are on' );
  third_party_code();
  is( $^W, 1, 'warnings are still on' );
[download]

That's only half of your tests, isn't it? What's missing is:

   {   local $^W;
       third_party_code();
       ok (!$^W, 'warnings still off');
   }
[download]

after all, you want to test that a piece of code leaves $^W as is.

But then, why stop at testing $^W? How about $*? $"? @ARGV?, %SIG? $package::var?

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