IMHO the proper solution isn't (or at least not only) rigorous testing, but escaping by default. I've asked about that in the past, Re: HTML::Template vs. Template::Toolkit vs. ?? (esp. the reference to Template::Stash::HTML::Entities and default_escape for Template::Toolkit? might be relevant.
If none of those solutions work well, I'd consider it sufficient reason not to use TT2, and switch to a template system that supports default escaping.
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