Perl is actually executing the text transformations. The python is a translation layer between the bsed syntax and the Perl command, so you get the performance of Perl with a very minor time cost for interpretation done in Python.
Yeah, there's not a lot of point in having the python layer. Perl excels at these kinds of things. I'm guessing that the original developer just doesn't know perl well enough to implement that part. Which is not to denegrate his work, but this could easily be a drop in single file perl executable if someone does the actual (not that much) work.