Declare the variables outside the subroutine

#!/usr/bin/env perl
use strict;
use warnings;
use utf8;
use feature "say";
use Math::Trig;

my ($radius,$c);

sub intro {
  say "What is the radius?";
  $radius = <STDIN>;
  chomp ($radius);
  my $a = 2*pi;
  $c = $radius * $a
}

intro();

if ($radius==0) {
   say "circumference= ", $radius;
}

elsif ($radius<0) {
    say "Radius should be greater than 0";
    intro();
}

else {
   say "circumference= ", $c
}
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Ahhh, now I understand. D'oh! :D

Note the missing semi-colon on one of the last lines there...

I do not understand what you mean by "submodule". Again, could you share the code you're speaking of?

We can help you understand not just how to do the loop, but the issues with the other code you have. Look at it as a learning exercise ;)