use strict;
use warnings;
my @array1 = (1..4);
my @array2 = (1..4);

for my $val1 (@array1) {
    my $outer_pid = fork();
    die "fork failed ($val1)" unless defined $outer_pid;
    if ($outer_pid == 0) {
        open my $fh, ">", "$val1.txt" or die "Open fail: $!";
        print $fh "Output of step 1\n";
        exit 0;
    } else {
        waitpid $outer_pid, 0;
        for my $val2 (@array2) {
            my $inner_pid = fork();
            die "fork failed ($val1,$val2)" unless defined $inner_pid;
            if ($inner_pid == 0) {
                 open my $fh1, '<', "$val1.txt" or die "Open fail: $!";
                 open my $fh2, ">", "$val2$val1.txt" or die "Open fail: $!";
                 while (my $line = <$fh1>) {
                     $line =~ s/1/2/;
                     print $fh2 $line . "\n";
                 }
                 exit 0;
             } else {
                 waitpid $inner_pid, 0;
             }
       }
   }
}