The pipeline style does all the work up front in a standard CGI or mod_perl handler, then decides which template to run and passes some data to it. The template has no control flow logic in it, just presentation logic, e.g. show this graphic if this item is on sale. Popular systems supporting this approach include HTML::Template and Template Toolkit.

The callback model works well for publishing-oriented sites where the pages are essentially mix and match sets of articles and lists. Ideally, a site can be broken down into visual ``components'' or pieces of pages that are general enough for an HTML coder to recombine them into entirely new kinds of pages without any help from a programmer.

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The pipeline model is more like a traditional model-view-controller design. Working this way can provide additional performance tuning opportunities over an approach where you don't know what data will be needed at the beginning of the request. You can aggregate database queries, make smarter choices about caching, etc. It can also promote a cleaner separation of application logic and presentation. However, this approach takes longer to get started with since it's a bigger conceptual hurdle and always involves at least two files: one for the Perl code and one for the template.