I noticed something similar on a page that compared Perl 5 and Perl 6 code. The surrounding text indicated which block was which, as well as pseudocode and expo-only stuff. I thought it could be color coded.
How about adding an optional property to the CODE tag? E.g. <CODE class=perl6> will show some styleistic change such as color or margin stripe. The actual meaning can be per-user, expanded in a style sheet.
In any case, I think a global 5/6 switch is not enough, since code can be mixed in one node.