Multiple columns from one table rely on one single table
I had to use two JOINs here but I think that this will work. You can join on the same table multiple times

SELECT 
  a.t_code,
  a.t_waste_water,
  a.t_drinking_water,
  b.perim_name AS waste_water_perim,
  c.perim_name AS drinking_water_perim

FROM schema.town a
  JOIN schema.perimeter b ON a.t_waste_water = b.perim_code 
  JOIN schema.perimeter c ON a.t_drinking_water = c.perim_code;