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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