I heard from maintainer Marquess. 

http://docs.oracle.com/cd/E17076_02/html/programmer_reference/am_misc_
+stability.html

[download]
explains that modifying a value in an each iteration on a tied hash is not allowed unless transactions are used.

In reply to Re^5: altering value in tied hash by rlmoe
in thread altering value in tied hash by rlmoe

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