MongoDB drops support for Perl 5.8.
One of my many rules of software engineering, born of more than a decade seeing things done the Wrong Way, is that serialization must occur only at the extreme edges of your program. At all other points you should, if possible, deal only with structured data. The lack of it in one crucial area of the Perl MongoDB driver is what made support for Perl 5.8 no longer possible.
Read more about my rationale for this change at my blog here: Structured Data and the Road to Obsolescence.