use Algorithm::Loops 'NestedLoops';

my $iter = NestedLoops([[\(@arr)], do {my $i=0; sub {[$i++]}} ]);

while (my ($line, $x) = &$iter) {
    for my $line ($$line) {
        print "$x: $line\n";
        $line = 'X';
    }
}

print "@arr\n";