Mathf.PI  

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

class example(MonoBehaviour):

    public radius as single = 5

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