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
{
  if(length($Test)>1)
  {
    $ent->configure(-foreground=>'red');
  }
  else
  {
    $ent->configure(-foreground=>'black');
  }
}