For my part, I have had no concerns about the plans for concurrency in Perl 6, at the language level, going back a very long time.

I don't follow how STM can be used to address locking of non-reversible external operations. There seems to be a total disconnect between the requirements and operational modes of the two:

STM is optimistic: every thread completes its modifications to shared memory without regard for what other threads might be doing, recording every read and write that it makes in a log. Instead of placing the onus on the writer to make sure it does not adversely affect other operations in progress, it is placed on the reader, who after completing an entire transaction verifies that other threads have not concurrently made changes to memory that it accessed in the past. This final operation, in which the changes of a transaction are validated and, if validation is successful, made permanent, is called a commit.

But even if it cannot, some other mechanism can be used. Provided that other mechanism is availabe to the language from the underlying runtime.

My concerns have been, and remain, whether Parrot will have the wherewithal to underwrite the demands and operations that Perl 6 will call upon it to make.