#!/usr/bin/perl --
use strict;
use warnings;
use Tk;

my $mw      = MainWindow->new();
my $textvar = 1;

my $e = $mw->Entry(
    -textvariable => \$textvar,
    -validate     => 'key',
);
$e->configure( -validatecommand => [ \&myValidate, $e ], );
$e->pack();
MainLoop;

sub _configurevalidate_key {
    $Tk::widget->configure( -validate, 'key' );
}

sub myValidate {
    my( $entry, $new, $changed, $old, $ix, $type ) = @_;
#~     $entry->afterIdle( [ \&_configurevalidate_key, $e ] );
    $entry->afterIdle( \&_configurevalidate_key  );
    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;
}

__END__