use strict;
use warnings;
use Tk;

my $mw = Tk::MainWindow->new();
my $entry = $mw->Entry()->pack();
my $var = '123';

# sub in []-klammern packen
$entry->bind('<KeyPress>', [sub{
        $entry->configure(-text => "$_[1]\n");
}, $var]);

$mw->MainLoop();