According to my copy of the Essential POTO, the punjab lasso was probably a reference to Eugene Sue's The Wandering Jew. Leroux was most likely familiar with this work, and there is a passage in it that describes a Thugee killing someone with a cord similar-ish to the punjab lasso. Although the description is plus like a bola than a lasso in that work, as the cord has weighted balls at each end. It is thrown and wraps around the victem's throat tightly, with the two balls striking the back of the head.
Another interesting notation on the punjab lasso that I have read in the past is that it is not meant to strangle, but to garrotte the victim. To garrotte is to cut off the flow of blood to the brain. It kills quicker than strangulation.
Also, wiki entry on the Punjab Region which is now part of both India and Pakistan.
link . There is also a Panjab region in Afghanistan, although the reference comes most likely from the Indian region.
