Also in the same chapter Parr writes:

Interestingly, by this definition of a template, XSL ... style sheets are not templates at all because style sheets specify a set of XSLT tree transformations whose emergent behavior is an XML or HTML document. XSL style sheets are programs like servlets, albeit declarative in nature rather than imperative.

And I was never absolutely sure where the real dinstinction is drawn here.

When your "traditional" templating system tends to become a Turing complete programming language, the distinction blurrs away anyway.

I like in XSLT that it won't break the tree structure of the "template" I'm working in (XML, XHTML). So typically for my taste it's a better fit. Which might only be exactly that.. a matter of taste.

The same might or might not apply to you and your HTML::Seamstress.