Can lying be disincentivized without serious curtailments of free speech?
If free speech is narrowly defined to only include true claims, true and false claims can be perfectly distinguished, the harm of false claims perfectly measured, and all claims are either true or false, then false claims are really no different than other forms of pollution. A standard pigovian tax should internalize the externalities and encourage the socially optimal level of truth telling. The social planner just has to tax each lie enough to discourage the social harm it does.

However, I maintain that in practice all of these assumptions are false. All experience seems to show that using the legal system to purge public discourse of lies leads to concentrating enormous power in those who make the lie / truth determination and that they will use that power in a way that is often inconsistent with objective truth And, when there is no objective truth there will still be those who would use the legal system to control public discourse in the name of protecting it.

In addition, the real world's imperfect legal processes are expensive and painful to participate in. That means that many true things that may be confused with lies will not be said, and that will further restrict freedom of speech.