You don't show us any code. You don't show us how you're writing the files. If you use a delimiter between the two files, it's far more likely that there is an error in your (non-socket) program logic that mixes up the content of the files. Also, I wouldn't bet on a single read corresponding to a single write on the other end.

Here is the code. I am new to perl programming. I have to spawn four sub processes but initially trying to get 1 process to send the files. So the same socket connection is used to send the data and wait till they give us response and receive the response and write to a file.

my $BUFSIZE = 32000;
my $varhex3= "\x03";

$SIG{USR1} = "doneit";
# I added the below for trial and error not sure what this #does.
my $nonblocking = 1;  
select(S);
$| = 1;
select(STDOUT);

for( my $childcounter = 1; $childcounter <= 1; $childcounter++ ) {
            
    my $pid = fork();
    if ($pid) {
    # parent
          
    push(@childs, $pid);
    } elsif ($pid == 0) {
    # child
            
    if ($childcounter == 1)    {
        @mylist1 = @{$hash1{$childcounter}};
        my $socket1 = &opensock();
        $myport = $socket1->sockport();
        foreach $File (@mylist1){
            
            $BaseFName = basename($File);
            my $outputdir = "directory goes here";
            my $outputfile = "$outputdir\/$BaseFName".".res";
            open(INFILE,"$File");
                   $data=<INFILE>;
                close INFILE;
#             print $sock "$data";
             $socket1->send ($data);
            open (RESPONSEFILE, ">$outputfile") or die $!;
             my $lastchar = "";
             my $Stop_Read = 0;
             while (! $Stop_Read) {
                  $lastchar = "";
                  $data1 = "";
                  recv($socket1,$data1,$BUFSIZE,0); 
                 sleep 2;
                 my $mylen = length($data1);
                 $lastchar = substr ($data1, $mylen-1, 1);
#                print STDOUT $data1;
                print RESPONSEFILE $data1;
                 if ($lastchar == $varhex3){
                $Stop_Read = 1;
                }
                            
            }
            close RESPONSEFILE;
            }
            close($socket1);
        }
}


sub doneit {
        $gotone = 1;
}

sub opensock{
my $sock = new IO::Socket::INET (
                                 PeerAddr => 'IP address',
                                 PeerPort => 'Port number',
                                 Proto => 'tcp',
                                 Listen => SOMAXCONN,
                     Reuse => 1,
                     Timeout => 60,

                                );
 die "Could not create socket: $!\n" unless $sock;
#select (sock); $| = 1;

}
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I am also encountering the issue where two successive writes are picked up by second read.

Not surprisingly, the same answer applies to you.