This is not something you are going to need to do everyday, and probably less and less as PLT files become a thing of the past. But, if you ever do need to, here are two tools that may help:
guthrie CAD :: GIS – Hpgl2CAD Download
15 day trial allows 10 translations with no file size limit.
HPGL2CAD 4.0
4.0 A.13
11.0 MB, Updated December 7, 2011
