In general, it is difficult to have Tk and fork in the same script. Not impossible, but difficult. You probably will need to fork the process before generating any Tk widgets.

Is it entirely necessary to fork though? Here is a simple forking socket server (copied from Perl interprocess comunications and lightly modified to send the local time 5 times a second.) Invoke it with the port to use: defaults to 2345.

    #!/usr/bin/perl
    use strict;
    BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
    use Socket;
    use Carp;
    use Time::HiRes;
    
    my $EOL = "\015\012";

    sub spawn;    # forward declaration
    sub logmsg { select STDERR; warn "$0 $$: @_ at ", scalar localtime
+, "\n" }

    my $port = shift || 2345;
    my $proto = getprotobyname('tcp');

    ($port) = $port =~ /^(\d+)$/ or die "invalid port";

    socket( Server, PF_INET, SOCK_STREAM, $proto ) || die "socket: $!"
+;
    setsockopt( Server, SOL_SOCKET, SO_REUSEADDR, pack( "l", 1 ) )
      || die "setsockopt: $!";
    bind( Server, sockaddr_in( $port, INADDR_ANY ) ) || die "bind: $!"
+;
    listen( Server, SOMAXCONN ) || die "listen: $!";

    logmsg "server started on port $port";

    my $waitedpid = 0;
    my $paddr;

    use POSIX ":sys_wait_h";
    use Errno;

    sub REAPER {
        local $!;    # don't let waitpid() overwrite current error
        while ( ( my $pid = waitpid( -1, WNOHANG ) ) > 0 && WIFEXITED(
+$?) ) {
            logmsg "reaped $waitedpid" . ( $? ? " with exit $?" : '' )
+;
        }
        $SIG{CHLD} = \&REAPER;    # loathe sysV
    }

    $SIG{CHLD} = \&REAPER;

    while (1) {
        $paddr = accept( Client, Server ) || do {

            # try again if accept() returned because a signal was rece
+ived
            next if $!{EINTR};
            die "accept: $!";
        };
        my ( $port, $iaddr ) = sockaddr_in($paddr);
        my $name = gethostbyaddr( $iaddr, AF_INET );

        logmsg "connection from $name [", inet_ntoa($iaddr), "] at por
+t $port";

        spawn sub {
            select Client;
            $| = 1;
            while (1) {
                my $time = scalar localtime;
                print Client "Hello there, $name, it's now $time $EOL"
+;
                Time::HiRes::sleep .2;
            }
        };
        close Client;
    }

    sub spawn {
        my $coderef = shift;
        unless ( @_ == 0 && $coderef && ref($coderef) eq 'CODE' ) {
            confess "usage: spawn CODEREF";
        }

        my $pid;
        if ( !defined( $pid = fork ) ) {
            logmsg "cannot fork: $!";
            return;
        }
        elsif ($pid) {
            logmsg "begat $pid";
            return;    # I'm the parent
        }

        # else I'm the child -- go spawn

        open( STDIN,  "<&Client" ) || die "can't dup client to stdin";
        open( STDOUT, ">&Client" ) || die "can't dup client to stdout"
+;
        ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr
+";
        exit &$coderef();
    }
[download]

Then use this script to connect to it and display the received data. (connect to localhost:port if the server is on the same machine or the servers IP:port if remote.)

This is not completely non-blocking, it updates 10 times a second though so it is pretty responsive.

UPDATE: This is not non-blocking at all :(. See updated script below.

#!/usr/bin/perl

use strict;
use warnings;
use diagnostics;

use Tk;
use Tk::Text;
use IO::Socket;

my ( $ip_addr, $line, $sock, $afterid, $cancel );

my $mw = new MainWindow;

# Put a Frame in it/give it a label
my $frm_name = $mw->Frame();
my $lab = $frm_name->Label( -text => "Enter IP address:port" );

# We need entry boxes for IP address:socket & a button to start runnin
+g.
my $ent1    = $frm_name->Entry();
my $but     = $mw->Button( -text => "Start", -command => \&start );
my $stopbut = $mw->Button( -text => "Stop", -command => \&stop );

# Create a window for Display and Input.

my $textarea = $mw->Frame();          #creating another frame
my $txt      = $textarea->Scrolled(
    'Text',
    -width      => 80,
    -height     => 10,
    -scrollbars => 'osoe'
);
$lab->grid( -row => 1, -column => 1 );
$ent1->grid( -row => 1, -column => 2 );
$frm_name->grid( -row => 1, -column => 1, -columnspan => 2 );

$but->grid( -row => 4, -column => 1 );
$stopbut->grid( -row => 4, -column => 2 );
$txt->grid( -row => 1, -column => 1 );

$textarea->grid( -row => 5, -column => 1, -columnspan => 2 );

MainLoop;

sub start {
    return if defined $afterid;
    $cancel  = 0;
    $ip_addr = $ent1->get();
    print STDOUT "addr: $ip_addr\n";

    $sock = new IO::Socket::INET(
        PeerAddr => $ip_addr,
        PeerPort => '5000',
        Proto    => 'tcp',
    );
    die "Could not create socket: $! \n" unless $sock;

    $afterid = $mw->after(
        100,
        sub {
            while ( defined( $line = <$sock> ) ) {
                $line =~ s/\015\012/\n/;
                $txt->insert( 'end', $line );
                $txt->see('end');
                $mw->update;
                last if $cancel;
            }
        }
    );
}

sub stop {
    return unless $afterid;
    $cancel = 1;
    $afterid->cancel;
    undef $afterid;
    close $sock;
    undef $sock;
}
[download]