I use CGI::Application and for the edification of like-minded folks, here's what I came up with (basically an array of paths, the 2nd one used for site-specific tmpls):

#instance script passes in PARAMS which is also the directory with sit
+e specific .tmpls

my $site = $self->param('name_of_site'); 

$self->param('project'    => '/usr/home/somedomain/ecomm/');
$self->param('tmpl_path' => [ $self->param('project_').'tmpls/',
                                  $self->param('project_path').'tmpls/
+includes/'.$self->param('site') ]);
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