Why are you forced to specify in advance the context that you'll be using?

Prior to threads v1.31, the context would be determined by how the ->create() call was made. This lead to the following counter-intuitive syntax if you wanted the threads object, but were expecting to get a list back from the thread:

    my ($thr) = threads->create('foo');
    ...
    my @results = $thr->join();
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With threads 1.31, you can now specify the context directly so as to avoid such confusion:

    use threads 1.31;

    my $thr = threads->create({'list'=>1}, 'foo');
    ...
    my @results = $thr->join();


    threads->create({'context'=>'scalar'}, 'bar');
    ...
    foreach my $thr (threads->list()) {
        my $rc = $thr->join();
    }
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Does threads throw an assertion error if you accidentally use a different context than the one you promised you'd use?

No, essentially it is so the the thread 'entry point' (the subroutine that forms the thread essentially) can have a different context than that of the create() which started the thread, that is to say you can tell it in advance what context the join() will be called in so that wantarray() will work properly, bearing in mind that the thread may already have checked wantarray to determine what to return before the context of the join is known.

/J\