"Southern rifle" is a pretty broad term. Rifles from the various regions of the south have different characteristics, but in general southern guns have iron furniture, usually walnut stocks, but sometimes maple or cherry, with more drop in the butt than say a Lancaster rifle.
A "Poor Boy", aka Barn Gun or Schimmel, could come from any region. They are very plain, usually made without a butt plate, toe plate, or ramrod entry pipe, and a washer instead of a large side plate.
I think the term Poor Boy is more often used in the south, while Barn Gun and especially Schimmel is more often used up north.
-Ron