Invalid value for shared scalar

I got this error, which I traced down to accidentally calling is() with
a hashref as the third argument, where the name should have been:

    use Test::More 'no_plan';
    is(1,1,{});

This happens because Test::Builder tries to assign the name to a key of
a shared hash.  This simple case demonstrates the failure:

    use threads;
    use threads::shared;

    share(%foo);
    $foo{bar} = {};

I found some mailing list discussions about this problem without much
conclusion.  This leads me to believe that perl threads are not mature
enough to foist on people without their express consent.  Of course,
this case can be fixed, but if such innocent code can be broken by
threads, I'm sure there are more bugs lurking.  And they are very
frustrating to debug for people without experience in perl threads.

So I suggest that Test::Builder not enable threads itself, unless
explicitly requested.  Even if your code uses threads, it seems unlikely
that you'd want to run your tests in parallel anyway.  You could perhaps
enable thread-safe tests with

    use Test::Builder 'thread_safe';

Andrew