Assuming this snake really does come from the Chaco, I hopefully narrowed the list down a bit for you. Doing a quick search under the EMBL database using Liophis and Paraguay and eliminating all the pictured snakes (hopefully there isn't too much geographic variation), I was left with the following species: anomalus, dilepis, frenatus, guentheri, miliaris, sagittifer, viridis. Hopefully one of these rings a bell. I've actually been wondering what species this is for some time since I saw a few of these guys at a show a few years back. If you still aren't sure, I'd check into guentheri a bit since EMBL states that it's found exclusively in the Chaco region where its range extends into Argentina. Hope this helps.
Regards,
Wes