use Tk;
  my $mw = MainWindow->new();
  my $textvar = 1;
  $mw->Entry(-textvariable => \$textvar,
             -validatecommand =>
               sub { my ($new,$changed,$old,$ix,$type) = @_;
                     return 1 if (!defined($changed));
                     return 1 if ($new eq "") or ($type<0);
                     $textvar = $new & 3; # use only 2 low bits
                     print "masked out entry value\n";
                     return 1;
                   },
             -validate => 'key')->pack();
  MainLoop;