[perl]Circumfrence of a circle

#!/usr/bin/env perl
#input radius
#output Circumfrence of a circle
print "2.11[1])\nPlease enter radius:";
$radius=<STDIN>;
if($radius >= 0){
        $out=$radius *2*3.141592654;
        print "Circumfrence of a circle is:$out\n";
}
else{
        print "Please enter postive real number.";
}