need program to terminate on "no" value

#!C:\Strawberry\perl\bin
use warnings;

until ($stop != 0) {

$stop = 0;

print "this program will compute the circumfrence of a circle.\n";
print "\n";
print "Please enter in the radius of your circle.\n";
$radius = <STDIN>;
until ($radius ne "\n") {
    print "please enter a number ";
    $radius = <STDIN>;
}

print "The radius of your circle is ";
print 2 * 3.141592654 * $radius;
print "\n";
print "do you want to calculate another?(yes/no)\n";
$answer = <STDIN>;
if ($answer eq "no") {
    print "exiting.... \n";
    $stop = 1;
    exit;
} else {
    $stop = 0;
}
}