in reply to Re^4: altering value in tied hash
in thread altering value in tied hash

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 Section Seekers of Perl Wisdom