It's off by default and I turned it on by overriding load_tmpl in CGI::Application, on the principle that one better be safe than sorry. I am finding however that I am spending a lot of time untainting stuff that ought to be secure, just so I can out it into HTML::Template.
Update I have tried to reformulate this question here.

In reply to Is the force_untaint option in HTML::Template overkill? by SilasTheMonk

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