use strict;
use warnings;
use Fcntl ":flock";

my @array1 = (1..4);
my @array2 = (1..4);

local $SIG{CHLD} = 'IGNORE';

for my $val1 (@array1) {
    my $pid = fork();
    die "fork failed ($val1)" unless defined $pid;
    if ($pid == 0) {
        open my $fh, ">", "$val1.txt" or die "Open fail: $!";
        flock($fh, LOCK_EX) or die "Cannot lock $val1.txt - $!\n";
        print $fh "Output of step 1\n";
        sleep 1;
        flock($fh, LOCK_UN) or die "Cannot unlock $val1.txt - $!\n";

        exit 0;
    }
}

for my $val1 (@array1) {
    for my $val2 (@array2) {
        my $pid = fork();
        die "fork failed ($val1, $val2)" unless defined $pid;
        if ($pid == 0) {
            open my $fh1, '<', "$val1.txt" or die "Open fail: $!";
            flock($fh1, LOCK_SH) or die "Cannot lock $val1.txt ($val2) - $!\n";
            open my $fh2, ">", "$val2$val1.txt" or die "Open fail: $!";
            while (my $line = <$fh1>) {
                $line =~ s/1/2/;
                print $fh2 "$line\n";
            }
            flock($fh1, LOCK_UN) or die "Cannot unlock $val1$val2.txt - $!\n";
            exit 0;
        }
    }
}