As I understand it, the problem related to Test::Builder will affect 5.8 and later because the current version of Test::Builder checks that perl version is greater then 5.008 before it enables this thread safety. From Test::Builder:

BEGIN {
    use Config;
    if( $] >= 5.008 && $Config{useithreads} ) {
        require threads;
        require threads::shared;
        threads::shared->import;
    }
    else {
        *share = sub { 0 };
        *lock  = sub { 0 };
    }
}
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Is this correct?