hmm... as noted above your question is vague, but considering this:

there must be a text/string based way (can't save A::D::diff output without serializing it) without files and without asking the shell for help.

I think you want to take a look at Text::Diff.

By the way, revisions in relational databases might be harder than you think, considering the "relational" part of the database. Diff et al only fix the "easy" part of the problem.