sub checksocket {
    my ($sock) = @_;
    my $data;
    $sock->read($data, $nnn)  # replace $nnn with number of bytes you want
             or return; # no data read
    # do something with $data here
}

my $mw = MainWindow->new;

# set up your Tk widgets here

my $sock = IO::Socket::INET->new(PeerAddr => 'host:port',
                           Proto => 'tcp',
                           Blocking => 0); # set non-blocking mode

my $after = $mw->repeat(500,  # set to appropriate number of milis
                       sub { checksocket($sock) } );

MainLoop;