It seems like what you're trying to do violates modularity in a way that'll cause problems. That is, somebody who wants to extend this to a new format should ideally be able to just whip up a DBI::Pretty::Foo and start using it in scripts. But with this design, any such developer also has to muck with the DBI::Pretty.pm as well, adding an elsif to that bit of (pseudo-)code. This is really undesirable, potentially making it difficult to maintain and use your module.

As an alternative, how about this:

• Instead of 'my $hash = shift || {}' (inside your constructor), create a parse_input() method. (Or grab_input(), or something similar.) In the case of your existing module, it would just consist of that one statement, returning a hashref.
• Subclasses handling different forms of input could now be created by simply replacing the new() and parse_input() methods with ones of their own construction, returning a hashref that conforms to your requirements. (Actually, you could make it so that people extending your module would only have to rewrite parse_input(), if you get creative with the way you write your new() method -- blessing an object immediately, and making all additional calls in new() through the -> operator, i.e. "$self->parse_input".)
• Anyone using the module must now decide in advance what form the input will take, and must explicitly choose the corresponding module ('use DBI::Pretty::XML').

If you do want the module to be flexible enough to handle a few alternative situations by default without requiring subclasses, just place that if/elsif you've got inside the default parse_input(), but rather than calling alternative use statements, each condition should just invoke different parse_foo_input() methods. Doing it like this -- in parse_input(), not in new() -- shouldn't interfere with the ability for other people to roll their own subclasses.

But the simplest/best thing is probably to just require that anyone using the module must themselves pick the subclass(es) that they want. You can then provide a few basic alternative subclasses along with the main module.

-- Frag.