Ha ha, trick question, there is no "best" templating system except for the one that's best for your project. Vince Veselosky has a roundup of Perl templating systems where he examines everything....
... from the Swiss Army Chainsaw of Template Toolkit, through HTML::Mason and Text::Template down to the ever-tempting "variables interpolated in a here-doc" method.... Read on for a comparison of the major template systems in Perl, and my recommendations of which systems fit which circumstances.
It's a fine introduction to the various systems, and probably worth pointing to from the Perl 5 wiki, if not reproducing it there entirely.
I also keep my templating comparison updated at http://perl.apache.org/docs/tutorials/tmpl/comparison/comparison.html. I need to update it again soon to add coverage of Template::Alloy, which has matured enough to include.
I am using Clearsilver and pretty happy with it.
It's C based, has a Data set approach to fill the template. Perl side is, well... Perlish. It doesn't have the whole C interface but with Inline::C it would be very easy to extend if one wants to use it.
I haven't used it in a mod_perl environment, fastcgi seems to be working.
If the application needs optimizations, one could easily create a Prototype with Perl, and then conver t to C using this prototype as a guide.
I am not sure but Clearsilver is being used in Google & Yahoo groups.
ClearSilver is not being used by Yahoo. According to Rasmus Lerdorf, they've replaced it with PHP. I did read something about Google Groups using it a few years ago.
Seems from what I have seen, the top 2 are TT and HT.