Working code for Client.pl is provided under:

#!/usr/bin/perl
use utf8;
use strict;
use warnings;
use Data::Dumper;
use IO::Socket::INET;
use constant ARGUMENTS => scalar 2;
use constant NICKNAME => scalar 12;
use constant MAXBYTES => scalar 255;

# flush memory after every initialization
$| = 1;

my $info = $ARGV[0]; # User message argv[0]
my $Nickname = $ARGV[1]; # User nickname argv[1]
my ( $kidpid, $line , $client_sock , $server_data , $send );
my $error = 'ERROR';
my $newline = "\n";

if (@ARGV > ARGUMENTS) {
    print "\nPlease no more than ".ARGUMENTS." arguments (ARGV[])!\n";
    print "\nCorrect Syntax: perl $0 'IP:PORT  NICKNAME' (e.g. 127.0.0
+.1:5000 Thanos)\n\n";
    exit();
}
elsif (@ARGV < ARGUMENTS) {
    print "\nPlease no less than ".ARGUMENTS." arguments (ARGV[])\n";
    print "\nCorrect Syntax: perl $0 'IP:PORT  NICKNAME' (e.g. 127.0.0
+.1:5000 Thanos)\n\n";
    exit();
}
else {
    
    my $string = index($info, ':');

    if ($string == '-1') {
    die "Please add ':' in your input - ".$info."\n";
    }

    my @input = split( ':' , $info );

    # create a tcp connection to the specified host and port 
    $client_sock = IO::Socket::INET->new( Proto    => "tcp",
                      PeerAddr => $input[0],
                      PeerPort => $input[1] 
    ) or die "Can't connect to port ".$input[1]." at ".$input[0].": $!
+\n";

    $client_sock->autoflush(1);    # so output gets there right away 
    print STDERR "[Connected to ".$input[0].":".$input[1]."]\n";
    
    $line = <$client_sock>;
    my $receive = &receive($line);
    #print "First receive: ".$receive."\n";
    if ($receive eq "Hello version") {
    $Nickname = "NICK ".$Nickname."";
    $send = &send($Nickname);
    #print "First send: ".$Nickname."\n";
    $line = <$client_sock>;
    $receive = &receive($line);
    #print "Second receive: ".$receive."\n";
    if ($receive eq "OK") {
        # split the program into two processes, identical twins
        print "Client '".$ARGV[1]."' enter your text here:\n";
        die "can't fork: $!" unless defined( $kidpid = fork() );
        # the if{} block runs only in the parent process 
        if ($kidpid) {
        # copy the socket to standard output 
        while ( defined( $line = <$client_sock> ) ) {
            $receive = &receive($line);
            print "Third receive: ".$receive."\n";
            print "Client '".$ARGV[1]."' enter your text here:\n";
        } # End of While reading (parent)
        } # End of if (parent)
        # the else{} block runs only in the child process 
        else {
        # copy standard input to the socket 
        while ( defined( $line = <STDIN> ) ) {
            chomp ($line);
            my $line = "MSG ".$line."";
            $send = &send($line);
            if ($line =~ /quit|exit/i) {
            $line = "Client request ".$line."";
            my $send = &send($line);
            kill( "TERM", $kidpid );    # send SIGTERM to child 
            }
        } # End of read and send
        } # End of else child
    } # End of if (OK)
    else {
        print "Did not Receive OK!\n";
        exit();
    }
    } # End of if (Hello version)
    else {
    print "Did not receive Hello version!\n";
    exit();
    }
} # End of else @ARGV

sub send {
    $_[0] = "".$_[0]."".$newline."";
    utf8::encode($_[0]);
    print $client_sock $_[0];
    chomp($_[0]);
    #print "The following data send to Server: (\ ".$_[0]." \)\n";
    #$client_sock->send($client_packet,MAXBYTES);
    return $_[0];
}

sub receive {    
    # we can read from socket through recv()  in IO::Socket::INET
    #$client_sock->recv($server_data,MAXBYTES);
    utf8::decode($_[0]);
    chomp($_[0]);
    #print STDOUT "The following data received form Server: (\ ".$_[0]
+." \)\n";
    return $_[0];
}
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Working code for Server.pl is provided under:

#!/usr/bin/perl
use utf8;
use strict;
use warnings;
use IO::Select;
use Data::Dumper;
use IO::Socket::INET; # Non-blocking I/O concept.
use constant ARGUMENTS => scalar 1;
use constant NICKNAME  => scalar 12;
use constant MAXBYTES  => scalar 255;

# flush memory after every initialization
$| = 1;

my $info = $ARGV[0]; # User message IP:PORT;
my $error = "ERROR";
my $newline = "\n";
my ( $client_data , $server_sock , $buf , $sock , $receive , $new_sock
+ , $send , $readable_handles , $nick );

if (@ARGV > ARGUMENTS) {
    print "\nPlease no more than ".ARGUMENTS." input!\n";
    print "\nCorrect Syntax: perl $0 IP:PORT (e.g. 127.0.0.1:5000)\n";
    exit();
}
elsif (@ARGV < ARGUMENTS) {
    print "\nPlease no less than ".ARGUMENTS." input!\n";
    print "\nCorrect Syntax: perl $0 IP:PORT (e.g. 127.0.0.1:5000)\n";
    exit();
}
else {
    
    my $string = index($info, ':');

    if ($string == '-1') {
    die "Please include ':' in your input - ".$info."\n";
    }

    my @input = split( ':' , $info );

    $server_sock = new IO::Socket::INET( LocalAddr => $input[0],
                     LocalPort => $input[1],
                     Proto     => 'tcp',
                     Listen    => SOMAXCONN,
                     Reuse     => 1 ) or die "Could not connect: $!";

    print "\n[Server $0 accepting clients at PORT: ".$input[1]." and I
+P: ".$input[0]."]\n";

    $readable_handles = new IO::Select();
    $readable_handles->add($server_sock);

    while (1) {
    
    (my $new_readable) = IO::Select->select($readable_handles, undef, 
+undef, 0);
    
    # conver string to array @$new_readable
    foreach $sock (@$new_readable) {
        
        # Check if sock is the same with server (e.g. 5000)
        # if same (new client) accept client socket
        # else read from socket input
        if ($sock == $server_sock) {
        $new_sock = $sock->accept()
            or die sprintf "ERROR (%d)(%s)(%d)(%s)", $!,$!,$^E,$^E;
        $readable_handles->add($new_sock);
        $send = "Hello version";
        $client_data = &send($send);
        print "First send: ".$client_data."\n";
        }
        else {
        $buf = <$sock>;
        $receive = &receive($buf);
        print "First receive: ".$receive."\n";
        my @text = split(/ / , $receive , 2);
        #print Dumper(@text);
        if ($text[0] eq "NICK") {
            if (length($text[1]) > NICKNAME) {
            $send = "".$error." Please no more than ".NICKNAME." chara
+cters as nickname!";
            $client_data = &send($send);
            $readable_handles->remove($sock);
            close($sock);            
            }
            elsif ($text[1] =~ s/\W//g) {
            $send = "".$error." Special characters detected in the nic
+kname, please remove them!";
            $client_data = &send($send);            
            $readable_handles->remove($sock);
            close($sock);
            }
            else {
            $nick = $text[1];
            $send = "OK";
            $client_data = &send($send);
            print "Second send: ".$client_data."\n";
            }
        } # End of if ($text[0] eq "NICK")
        elsif ($text[0] eq "MSG") {
            if (length($text[1]) > MAXBYTES) {
            $send = "".$error." Please remember that message limit is 
+".MAXBYTES."";
            $client_data = &send($send);
            print "In case of message over ".MAXBYTES." send: ".$clien
+t_data."\n";
            }
            else {
            print "Second receive: ".$text[1]."\n ";
            # Get all client(s) socket(s)
            my @sockets = $readable_handles->can_write();
            # Send the same message to client(s)
            foreach my $sck (@sockets) {
                my $final = "".$text[0]." ".$nick." ".$text[1]."";
                utf8::encode($final);
                print $sck "".$final."".$newline."";
                print "Third send: ".$final."\n";
                #print STDOUT "The following data send to Client(s): (
+\ ".$buf." \)\n";
            } # End of foreach
            }
        } # End of elsif ($text[0] eq "MSG")
        else {
            print "Closing client!\n";
            $readable_handles->remove($sock);
            close($sock);
        } # End of else condition
        } # End of else condition ($sock == $server_sock)
    } # End of foreach new sock
    } # End of While (1)

    print "Terminating Server\n";
    close $server_sock;
    getc();
} # End of else @ARGV

sub send {
    $_[0] = "".$_[0]."".$newline."";
    utf8::encode($_[0]);
    print $new_sock $_[0];
    chomp ($_[0]);
    #print "The following data send to Cliets: (\ ".$_[0]." \)\n";
    #$client_sock->send($client_packet,MAXBYTES);
    return $_[0];
}

sub receive {
    #$new_sock->recv($client_data,MAXBYTES);
    utf8::decode($_[0]);
    chomp ($_[0]);
    if($_[0] =~ /^$/) {
    print "Data packet received empty!\n";
    print "From host: ".$new_sock->peerhost()." and port: ".$new_sock-
+>peerport()."\n";
    return $_[0];
    }
    elsif ($_[0] !~ /^$/) {
    #print STDOUT "The following data received from Client: (\ ".$buf.
+" \)\n";
    #print "From host: ".$new_sock->peerhost()." and port: ".$new_sock
+->peerport()."\n";
    return $_[0];
    }
    else {
    $error = "".$error."".$newline."";
    utf8::encode ($error);
    $server_sock->send($error);
    print "Invalid client: ".$new_sock->peerhost()." terminating!\n";
    $readable_handles->remove($sock);
    close($sock);
    }
}
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