#!/usr/bin/perl -w
#As usual there are 1000 ways to do this.
#one is to define $pi or the constant PI, eg. as follows 
#use constant PI => 4*atan2(1,1);#or use a module
use Math::Trig; #allows to use the Math::Trig module that is part of perl
$circumference=0; #reset variables
$radius=2;
#$circumference= $radius*PI;
$circumference= 2*$radius*pi;
print "$circumference\n";