In my experience, one killer reason for using Template::Toolkit is Jemplate (also on CPAN).

With this, you can use exactly the same templates server-side as client-side, and this provides a convenient way of achieving dynamic effects without writing them twice.

I like the option of building static copies of the site with ttree and tpage, which is useful for high traffic sites that just refresh content every N minutes instead of dealing with a Perl interpreter for each connection. Of course, you can do this with plain Perl, but I find Template::Toolkit just a bit easier.

p.s. Your example seems a little overcomplicated, wouldn't this do the job instead?

<td>[% book.authors.join(', ') %] ([% book.authors.size %])</td>
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