So what are the difficulties I've faced so far ?
For the Configure script, I've taken the easy way : no brutal reintegration of the modern units, only tweaks to solve known problems. (As Raphael Manfredi is apparently working on dist again, 5.6.3 will probably benefit from this.)
The modules are not difficult to upgrade, if they already work with 5.6.1.
The difficulty, not very surprisingly, comes from tests : the core test suite has been improved and reorganized in perl 5.8. Test files have moved. Smokes have started. Test targets in the main makefile have been added and made more complex. Some tests (for example, the tests for the newest MakeMaker) now use Test::More, which is not part of 5.6.1. That means that I'll probably have to borgify Test::More (and its test suite) in 5.6.2.