This might help you. It changes color, but dosn't flash. Also it helps to use 'any' for the validate method, rather than focusout. The key lines are 
$mw->waitVisibility;                                  
$mw->after(100,sub{$entries{3}{'entry'}->focusForce});

[download]
Here is a working script that should show you the way of highlighting a textvariable which is invalid. You can perfect it yourself. :-) 
#!/usr/bin/perl
use strict;
use warnings;
use Tk;

my $mw = MainWindow->new(-title => 'Mac Address Input Example');
my $fr = $mw->Frame()->pack(-expand => 1, -fill => 'both');
my $lb = $fr->Label(-text => 'MAC Address')->pack(-side => 'left');

my %entries;

for(1..6){
  $entries{$_}{'entry'} = $fr->Entry();
  $entries{$_}{'entry'}->configure(
    -textvariable    => \$entries{$_}{'addy'},
    -width           => 3,
#    -validate        => 'focusout',
#    -validatecommand => [ \&validate,$_,\%entries ],
#    -invalidcommand  => [ \&show_invalid,$entries{$_}{'entry'},$_ ],
  );
  $entries{$_}{'entry'}->pack(-side => 'left');

}

for(1..6){
$entries{$_}{'addy'} = 'zz'.$_ if($_ == 3);
  $entries{$_}{'entry'}->configure(
    #-validate        => 'focusout',
    -validate    => 'all',
    -validatecommand => [ \&validate,$_,\%entries ],
    -invalidcommand  => [ \&show_invalid,$entries{$_}{'entry'},$_ ],
  );
}

$mw->waitVisibility;
$mw->after(100,sub{$entries{3}{'entry'}->focusForce});

MainLoop();

# returns `1' if valid, and `0' if invalid
sub validate {
  my($num,$val) = @_;
  unless($val){ print "Field $num has nothing to validate\n";return 1;
+}

  my $valid = ($val =~ /^[0-9a-f]{1,2}$/i) ? 1 : 0;
  printf "Field %s, validating \`%s'...%s\n",$num,$val,$valid? "ok": "
+FAILED";

  return $valid;
}

sub show_invalid {
  my($widget) = @_;
#  print "@_\n";
  $widget->focus();

  # background of the problem field in red
  my $bg = $widget->cget('-bg');
  $widget->configure(-bg => 'lightpink');
  $widget->update();

}

[download]
I'm not really a human, but I play one on earth.
Old Perl Programmer Haiku ................... flash japh

In reply to Re^3: Tk::Entry and double-Tab key weirdness by zentara
in thread Tk::Entry and double-Tab key weirdness by atreyu

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