use Tk;
use utf8;
use strict;
use warnings;

my($Test, $Laenge, $Meldung) = "";

my $mw = MainWindow->new();
my $ent = $mw->Entry(
    -textvariable    => \$Test,
    -validate        => 'key',
    -validatecommand => \&check,
)->pack(-side=>'top');


MainLoop();


sub check {

    my $color      = 'black';
    my $returncode = 1;

    if ( 1 < length $Test ) {
        $color = 'red';
    }

    $ent->configure( -foreground => $color );
    return $returncode;
}