What is the perimeter of a sector?
The perimeter of the sector includes the length of the radius $\times 2$, as well as the arc length. So the perimeter is the length "around" the entire sector, the length "around" a slice of pizza, which includes it's edges and its curved arc.
The arc length is just the curved portion of the circumference, the sector permimeter is the length of line $\overline{AC} = r$ plus the length of line $\overline{BC} = r$, plus the length of the arc ${AOC}$.
The circumference of the circle is the total arc length of the circle.
Length is one-dimensional, the length of a line wrapped around the circle. Area is two dimensional; All of what's inside the circle.