Earlier today I posted that the Yerushalmi has a different reason than the Bavli for the din that eidim zomimim do not need hasra'ah. The Yl"mi writes that everyone knows that eidim who lie get punished, yet that does not deter anyone from testifying falsely, so hasra'ah would be pointless.
My son tried to one-up me yet again by observing that not only is the Yl"mi different than the Bavli, but it is also against the Bavli. The Bavli in its hava amina considers giving hasra'ah to be a practical impossibility. If you give hasra'ah after the eidim testify, it's too late -- testimony cannot be retracted. If you give the hasra'ah before they testify, then witnesses may not be willing to testify at all. Looking at the hava amina, it is clear that the Bavli thought hasra'ah given before testimony would be a very strong deterrent - not only would it discourage lying, it would discourage testimony completely.
The truth is that this observation is only partially correct. The Bavli also accepts that hasra'ah will not deter liars from testifying. The problem the Bavli raises is that hasra'ah is so powerful that it will discourage those who tell the truth from testifying. As Rashi puts is, "parshi v'lo m'sa'hadi - afilu emes..." Hasra'ah becomes like gun control laws -- they don't keep guns out of the hands of the criminals, but rather out of the hands of those who obey the law. Hasra'ah is not effective on the bad guys, but it will scare an honest citizen away from testifying.