Iterator Operations

If the iterator returns the list items in some canonical order, you can do this:

        sub both {
          my ($it1, $it2) = @_;
          my ($a, $b) = ($it1->(), $it2->());

          sub {
            return undef unless defined $a || defined $b;
            my $rv;
            if ($a lt $b || ! defined $b) {
              $rv = $a;
              $a = $it1->();
            } if ($b lt $a || ! defined $a) {
              $rv = $b;
              $b = $it2->();
            } else {          # $a eq $b
              $rv = $a;
              ($a, $b) = ($it1->(), $it2->());
            } 
            return $rv;
          }  
        }

This function works for any iterators that return items in alphabetical order
If an iterator represents a database query, this is the OR operation