Running t/db_dependent/Letters/TemplateToolkit.t results in data modification. Probably caused by the combination of AutoCommit==0 and txn_do in Patron/Modification.
Hmm. The problem seems not to be Patron::Modification, but the AddReturn calls triggering a txn_do. See bug 17964.
Closing this separate report. The problem should be solved on 17964.