#!/usr/bin/env perl

use strict;
use warnings;
use feature 'say';
use constant PI => 4 * atan2 1, 1;

say 'Type radius (>=0) or just hit Return to exit.';

while (1) {
    print 'Radius: ';
    chomp(my $radius = <>);
    last unless length $radius;

    if ($radius =~ /^[+-]?(?:\d+|\d*[.]\d+)$/) {
        if ($radius < 0) {
            say "Radius [$radius] is less than zero. Try again.";
            next;
        }

        say 'Circumference: ', 2 * PI * $radius;
    }
    else {
        say "Invalid radius [$radius]. Try again.";
        next;
    }
}