while( defined( my $item = $iter1->() ) {
    print "1: $item\n";
}

while( defined( my $item = $iter2->() ) {
    print "2: $item\n";
}
while( defined( my $item = $iter2->() ) {
    print "3: $item\n";
}

#outputs:
1: 1
1: 2
1: 3
2: y
2: n
3: 7
3: 14
3: 21
3: 28