async BLOCK;
    "async" creates a thread to execute the block immediately following
    it. This block is treated as an anonymous subroutine, and so must
    have a semicolon after the closing brace. Like "threads->create()",
    "async" returns a *threads* object.

$thr->error()
    Threads are executed in an "eval" context. This method will return
    "undef" if the thread terminates *normally*. Otherwise, it returns
    the value of $@ associated with the thread's execution status in its
    "eval" context.