use v6;
sub MAIN ( $in_path1, $in_path2, $out_path = 'CONCATENATED_FILES' ) {
    my $in1 = open $in_path1, :r;
    my $in2 = open $in_path2, :r;
    my $out = open $out_path, :w;

    for $in1.lines Z $in2.lines -> $line1, $line2 {
        $out.say: "$line1\t$line2";
    }

    .close for $out, $in1, $in2;
}