Rather than try to identify or fix the problem, just stick everything on one line.
It seems that Sereal leaves decoded hash iterators in an undefined state. This could be considered a bug in Sereal, but it's probably safest not to rely on having reset iterators after reading in the DB.
In 5.8.[2-9] there is an error on non-threaded perls with "can't locate structure for statement", and I can't be bothered track it down and fix or work around it. Let's just bump the minimum recommended version to 5.10, and keep 5.6.1 and 5.8 as "best effort".