Iterator Operations

        sub mingle {
          my @iterator = @_;
          my $next = 0;
          sub {
            my $line = $iterator[$next]->();
            $next = ($next + 1) % @iterator;
            $line;
          }
        }

Now if you have several iterators you can turn them into one:

        $new_it = mingle($it1, $it2, $it3);

(Proper handling of EOF conditions is left as an exercise.)