Thank you,but :D it still doesn't work

I did what you said but now it gives me the files like this :

jacky@ubuntu:~/Desktop/test$ cat 1
1jack
1john
1joe

jacky@ubuntu:~/Desktop/test$ cat 2
2jack
2john
2joe

And so on...Any fix to this too? , Jack
[download]

The following might help. I don't understand why you are forking, so I have not done so, but you could add forking inside the loop if you want to, as long as you increment and reset $index_y in the main process so that it is coordinated across all the child processes.

#!/usr/bin/perl
use strict;
use warnings;

open(my $y, "<", "y") or die "y: $!";
my @y = <$y>;
close($y);

my $index_y = 0;

open(my $x, "<", "x") or die "x: $!";

foreach (<$x>) {
    chomp;
    open(my $output, ">", "$_") or die "$_: $!";
    print $output "$_$y[$index_y]";
    close($output);
    $index_y++;
    $index_y = 0 if($index_y > $#y);
}

close($x);
[download]

That's my understanding of what you wanted. What output do expect from cat 1 and cat 2?