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