The infamous 3.14159265358979... value (Read Only).

	var radius : float = 5;
	function Start() {
		var perimeter : float = 2 * Mathf.PI * radius;
		Debug.Log("The perimeter of the circle is: " + perimeter);
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    public float radius = 5;
    void Start() {
        float perimeter = 2 * Mathf.PI * radius;
        Debug.Log("The perimeter of the circle is: " + perimeter);
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	public radius as float = 5

	def Start() as void:
		perimeter as float = ((2 * Mathf.PI) * radius)
		Debug.Log(('The perimeter of the circle is: ' + perimeter))