Thank you but it doesn't work...Maybe I'm doing something wrong :

#!/usr/bin/perl
use Parallel::ForkManager;

my $pm = new Parallel::ForkManager(3);

my @y;
{
   open my $fh_y, '<', 'y' or die $!;
   chomp( @y = <$fh_y> );
}

open(my $fh_x, '<', 'x') or die $!;
while (<$fh_x>) {
    $pm->start and next;
    chomp( my $linex = $_ );
    open my $fh_out, '>', $linex or die $!;
    for (@y) {
       print $fh_out ("$linex$liney\n");
    }
    $pm->finish   
}

When I perl script.pl it gives me the 5 "$filex" files but like this :

jacky@ubuntu:~/Desktop/test$ cat 1
1
1
1

So definatelly there is something wrong in there...Thank you again