Ah yes, I forgot about that. Revised answer:

Syntax. async is shorter, and it simply pass args via capture. That's good, because passing args to a thread as arguments is known to be problematic*.

Without args:

async(\&process);
async { process(); };
thread->create(\&process);
[download]

With args:

async(\&process, $arg);           # Safe??
thread->create(\&process, $arg);  # Safe??

async { process($arg); };
thread->create(sub { process($arg) });
[download]

* — The stack isn't ref-counted, and this causes issues when thread creation attempts to clone the variables. I can't remember the exact scenario, but I can look it up if you want more info.