#!/usr/bin/env perl

use strict;
use warnings;
use utf8;
use feature "say";
use Math::Trig;

&ask;

sub ask{
  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";
      &ask;
  }

  else {
     my $a = 2*pi;
     my $c = $a * $radius;
     say "circumference= ", $c
  }
}
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