I'll throw another into the mix...

#!perl
use warnings;
use strict;

use Tk;

my @mac = ( 32, 42 );

my $mw = MainWindow->new();

my $font = 'Courier 18 bold';

my @mac_entry;
for ( 0 .. 5 ) {

    $mac_entry[$_] = $mw->Entry(
        -textvariable    => \$mac[$_],
        -width           => 2,
        -font            => $font,
        -validate        => 'all',
        -validatecommand => \&valid,
        -invalidcommand  => [ sub { invalid( $_[0] ) }, $_ ]
    )->pack( -side => 'left' );

    $mac_entry[$_]->bind(
        '<KeyRelease>',
        [
            sub {
                if ( $mac[ $_[1] ] =~ /^[0-9a-f]{2}$/i ) {
                    $mac_entry[ $_[1] + 1 ]->focus
                      if defined $mac_entry[ $_[1] + 1 ];
                }
            },
            $_
        ]
    );
}

sub valid { $_[0] =~ /^[0-9a-f]{0,2}$/i }

sub invalid {
    my $index  = shift;
    my $widget = $mac_entry[$index];
    $widget->bell;
    my $background = $widget->cget('-background');
    $widget->configure( -background => 'red' );
    $widget->update;
    $widget->after( 250,
        sub { $widget->configure( -background => $background ) } );
}

$mac_entry[0]->focus;

MainLoop;
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