Just use Tk::fileevent.

Update: or try something like this:

use File::Tail;
use Tk;

use warnings;
use strict;

my $filename = "$ENV{HOME}/.xsession-errors";
my $log = File::Tail::->new(name => $filename,
                            interval => 1,
                            nowait => 1,
                           );


my $mw = MainWindow->new(-title => "Log Watch");
my $t  = $mw->Scrolled("Text")->pack;

$t->insert("end","== Started at ".localtime);


$mw->repeat(1000, sub {
                my $line = $log->read;
                $t->insert("end",$line) if defined $line;
            });

MainLoop;
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