I noticed a few things:
I tested the program using mIRC. I logged into the channel with the client and under another name with mIRC. Logging in works fine, I can see the clients nickname in the channel. What the problem seems to be is when the MOTD is displayed. It seems that only a few lines of the MOTD are displayed before the 'in' condition is stopped and the message stops displaying in the terminal window. When I type in mIRC, a line of the MOTD is displayed in terminal window. The $input variable seems to be behind the real-time sending of the messages. I'm not sure how to remove this lag and I'm looking for ideas.

#!/usr/local/bin/perl -w
use strict;
use Gtk2 '-init';
use Glib qw/TRUE FALSE/;
use IO::Socket;

#-------------------Shared Variables-------------------
my $server = "irc.freenode.net";
my $nick = "simple";
my $login = "simple";
my $channel = "#GRRUVI";
my $sock;

#-------------------Main Loop-------------------
my $window = Gtk2::Window->new('toplevel');
$window->signal_connect( delete_event => sub {
    close($sock);    
    Gtk2->main_quit;
});

$window->set_default_size( 300, 200 );

my $table = Gtk2::Table->new(2, 1, FALSE);
my $scroller = Gtk2::ScrolledWindow->new;
my $textview = Gtk2::TextView->new;
my $entry = Gtk2::Entry->new;

$scroller->add($textview);

$table->attach_defaults($scroller, 0, 1, 0, 1);
$table->attach_defaults($entry, 0, 1, 1, 2);

$window->add($table);

# allows for sending each line with an enter keypress
my $send_sig = $entry->signal_connect ('key-press-event' => sub {
        my ($widget,$event)= @_;
        if( $event->keyval() == 65293){  # a return key press
                my $text = $entry->get_text;
                if(defined $sock){ print $sock "PRIVMSG $channel :$tex
+t\r\n";}
                $entry->set_text('');
                $entry->set_position(0);
        }
});

$entry->signal_handler_block($send_sig); #not connected yet
$entry->set_editable(0);

$window->show_all;

connecting();

Glib::IO->add_watch( fileno $sock, [qw/in hup err/], \&incoming_data, 
+$sock );

Gtk2->main;

#-------------------Connect to Server-------------------
sub connecting {
    # Connect to the IRC server.
    $sock = new IO::Socket::INET(
        PeerAddr => $server,
        PeerPort => 6667,
        Proto => 'tcp'
    ) or die "Can't connect\n";
    
    # Log on to the server.
    print $sock "NICK $nick\r\n";
    print $sock "USER $login 8 * :Perl IRC Hacks Robot\r\n";

    # Read lines from the server until it tells us we have connected.
    while (my $input = <$sock>) {
        # Check the numerical responses from the server.
        if ($input =~ /004/) {
            # We are now logged in.
            last;
        }
        elsif ($input =~ /433/) {
            die "Nickname is already in use.";
        }
    }
    # Join the channel.
    print $sock "JOIN $channel\r\n";
    
    $entry->set_editable(1);
    $entry->grab_focus;
    $entry->signal_handler_unblock ($send_sig);
    
    Gtk2->main_iteration while Gtk2->events_pending;
}

#-------------------Incoming data-------------------
sub incoming_data {

    my ( $fd, $condition, $fh ) = @_;
  
    if ( $condition eq 'in' ) {
        my $input = scalar <$fh>;
        chop $input;
        if ($input =~ /^PING(.*)$/i) {
            # We must respond to PINGs to avoid being disconnected.
            print $sock "PONG $1\r\n";
        }
        else {
            # Print the raw line received by the bot.
            print "$input\n";
        }
    }
    return TRUE;
}
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