loop problem

[WisDomSeeKer34]

The program tries to calculate the circumference of a circle, depending on the input.

#!/usr/bin/env perl

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

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";
}

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

The problem is that if the value is lower than zero, the program stops. And I want it to continue till there is a positive answer or zero.

I tried it with a submodule, but I have to switch off the strict command to make it work.

It should be easy of course, but any help is greatly appreciated.