#!/usr/bin/env perl

use strict;
use warnings;
use feature "say";
use Math::Trig;

CALC: {
    say "What is the radius?";
    my $radius = <STDIN>;
    chomp($radius);
    if ( $radius == 0 ) {
        say "circumference= ", $radius;
    }
    elsif ( $radius < 0 ) {
        say "Radius should be greater than 0";
        goto CALC;
    }
    else {
        my $a = 2 * pi;
        my $c = $a * $radius;
        say "circumference= ", $c;
    }
}

__END__