$cmd = "tail -f /var/log/messages";
$fh = new IO::File;
my $pid = $fh->open ("$cmd|");
die "can't fork: $!\n" unless defined $pid;
Glib::IO->add_watch (fileno $fh, [qw/in hup err/],\&watch_callback, $fh);

sub watch_callback {
    my ($fd, $condition, $fh) = @_;
    if ($condition >= 'in') {
        my $data = scalar <$fh>;
        if (defined $data) {
            my $buffer = $textview1->get_buffer;
            $buffer->insert ($buffer->get_end_iter, $data);
        }
    }
    if ($condition >= 'hup' or $condition >= 'err') {
        $fh->close;
        $fh = undef;
    }

    if ($fh) {
        return TRUE;
    } else {
        return FALSE;
    }
}

$main->repeat(1000, sub { $log = qx(tail -f /var/log/messages) });

$main->Label(-textvariable => $log)->pack();