Version 5 features W3C-compliant HTML, much cleaner organization of the Perl code, and more dynamic content features including the weblog system and dynamic navigation menu.
Version 5.4 fixed some RSS-related stuff, some localization cleanup, and minor improvements to the Plain theme (including fixing some broken HTML).
Version 5.3 introduced live searching for the blog, with a few code changes to support that.
Versions 5.1 and 5.2 undoubtedly improved something, but I failed to keep good notes and these versions were never released.
Version 5.07 had minor code cleanup and new welcome graphics in more languages.
Version 5.06 was mostly a minor cleanup of variable names and HTML.
Version 5.05 included improved weblog search features, and minor code cleanup all over. It also included some graphics that were missing from previous releases. A changelog is now included.
Note that this release is intended for Perl hackers. If you don't know Perl, you may not be able to get a working site set up, and if you somehow manage to get it working, you probably won't enjoy getting there. Documentation is pretty much nonexistant.