I thought we had got AUTO_FILTER into Template::Toolkit. Oh - no, it was that somebody else wrote a module doing just that. A number of years ago on Perlmonks somebody else (not me) asked for the same thing, and then created Template::AutoFilter. I am the author of Template::Alloy (a near drop in for Template::Toolkit) and also added a native configuration item to Template::Alloy called AUTO_FILTER at about this same time. It is pretty easy to use in Alloy, so I assume that it would be easy to use in Template::AutoFilter as well.

use Template::Alloy;
my $t = Template::Alloy->new(AUTO_FILTER=>"html");
$t->process(\qq{[% foo %]\n}, {foo => "(&)"})'

(&)
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This excerpt comes from the Template::Alloy pod http://search.cpan.org/~rhandom/Template-Alloy-1.020/lib/Template/Alloy.pod.

AUTO_FILTER

    Can be the name of any filter. Default undef. Any variable returne
+d by a GET directive (including implicit GET) will be passed to the n
+amed filter. This configuration option may also be passed to the CONF
+IG directive.

        # with AUTO_FILTER => 'html'

        [% f = "&"; GET f %] prints &
        [% f = "&"; f %]     prints & (implicit GET)

    If a variable already has another filter applied the AUTO_FILTER i
+s not applied. The "none" scalar virtual method has been added to all
+ow for using variables without reapplying filters.

        # with AUTO_FILTER => 'html'

        [% f = "&";  f | none %] prints &
        [% f = "&"; g = f; g %]  prints &
        [% f = "&"; g = f; g | none %]  prints & (because g = f is a S
+ET directive)
        [% f = "&"; g = GET f; g | none %]  prints & (because the 
+actual GET directive was called)
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Update: I realized that this is one solution, but it is not a direct answer to the ops question. See my next reply for the real answer to the ops question.