Joel Feinberg, in his stunning Harm to Others (Volume 1 of his four volume The Moral Limits of the Criminal Law), provides a definition of liberalism I can strongly identify with.

We can define liberalism in respect to the subject matter of this work as the view that the harm and offense principles, duly clarified and qualified, between them exhaust the class of morally relevant reasons for criminal prohibitions. Paternalistic and moralistic considerations, when introduced as support for penal legislation, have no weight at all. (p. 14)

Feinberg then spends the next few thousand pages, over the course of four books, defending this view, with his usual collection of nuance, topical examples and thoughtfulness.

I don’t often associate with labels or principles – but, if forced to, I’d called myself a liberal in this, specific sense; it would only be of the Feinberg variety (which is a kind of modern, refined Millian take).

Feingberg doesn’t think criminal law is or should be entirely premised on “harm” as Mill and most others understand it; but he doesn’t think it should be based on other things either that are common, such as offence, immorality (loosely defined), and so on. He wants substanial proof that an act is actually harmful and in a significant way, before asking for criminal prosecution; indeed, even then, Feinberg says we should look for alternatives to prosecution and incarceration, if such alternatives exist and are demonstrably more effective.

We shouldn’t be defaulting to criminal responses and punishment, since we do that too often and can do too much and hurt too many. Indeed, as Feinberg highlights, this could itself be immoral: a good example is criminalising marijuana (and indeed most drugs) possession, which creates more harm as a response than the initial crime.