package MySub1;

use MySub2;

sub AAA {
  my $x = MySub2::DDD();
  return "AAA : $x";
}

sub BBB {
  return "BBB";
}

1;

##</code><code>##

package MySub2;

use MySub1;

sub CCC {
  my $x = MySub1::BBB();
  return "CCC : $x";
}

sub DDD {
  return "DDD";
}

1;

##</code><code>##

use strict;

use MySub1;
use MySub2;

print MySub1::AAA(), "\n";
print MySub1::BBB(), "\n";
print MySub2::CCC(), "\n";
print MySub2::DDD(), "\n";