This cannot work - if you expect ->lock() to block until the semaphore is released, how can it continue to execute the remaining code elsewhere?

The same way that $condvar->recv() blocks and lets code run elsewhere.

Only problem: $condvar->recv() cannot be called from within a callback, according to the AnyEvent docs:

"Note that doing a blocking wait in a callback is not supported by any event loop, that is, recursive invocation of a blocking "->recv" is not allowed,".