I use 5.8.4 on my workstations at work and home (both Debian Sarge).

All the production machines I target at work currently are running 5.6.1 (Debian Stable).

As the maintainer of XML::Simple, I get a lot of emails about basic Perl issues that have nothing to do with XML. The latest version of XML::Simple does not work with anything before 5.6 (Unicode support) and I get very few emails about that.