The projection labelled "Panini" in libpano13 is not actually the standard Pannini (cylindrical stereographic) projection, but a variant having an exaggerated vertical scale. It is not very useful and should probably be removed from the library.
For true Pannini projections, select "equirectangular Panini" or "Panini general". The latter, available since early 2010, is identical to the former at default parameter settings, and is adjustable for a wide range of perspective effects.
For the similarly named viewer Panini perspective tool by tksharpless see Panorama Viewers.