Internal vs external perimeter  

RE: Internal vs external perimeter

You can add a modifier to the model and set the number of perimeters to a higher number

and get something like this 

if you make the modifier a little smaller you can probably get fewer perimeters if you prefer. 
Please find a project file for demonstration below

Differential perimeters

I think you might have a misunderstanding  regards the naming protocol for perimeters

In 'Prusa Slicer' terminology, External Perimeters are the boundary between the model and air (making the external surface), internal perimeters are the interface between the external perimeters and the infill
In the image above the Key to the top left shows different colours for the internal and external perimeters

(AND NO... I don't know why the top image with differential number of perimeters shows external perimeters between the internal perimeters... ) lol... it just does...

now, if you want to have re enforced hole with less perimeters showing on the surface, you can shorten the modifier block, so that it does not extend past the ends of the block (Which may look nicer when printed) 
Please see the Project file, below 
Differential core perimeters

I have tried to show the inside and outside of the differential core, model as an  example  of the second Project file after slicing

regards Joan