I wouldn't expect a 5.8 application of any substance to work unmodified on perl 5.10.

I would. Looking through http://perldoc.perl.org/perldelta.html#Incompatible-Changes, there are a number of previously deprecated things removed and a number of changes that are basically bug fixes (where the new behavior is less surprising than the old), and very little else (depending on how much you personally place in the second category).