Are you sure you can't just use the PROCESS directive to process another template file?

The Template::Stash object appears to be accessible like this:

my $tt = Template->new();
my $stash = $tt->context()->stash();
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Though I imagine you'd want to weaken the reference before passing it to process with weaken from Scalar::Util. The plugin might be a more elegant solution... [% PROCESS ... %] even more so.