use strict;
use warnings;
use Tk;

my $mac_addr = "";

my $mw = new MainWindow(-title => 'Enter a MAC address');
my $fr = $mw->Frame()->pack(-expand => 1, -fill => 'both');
my $lb = $fr->Label(-text => "Enter MAC address ")->pack(-side => 'left');
my $en = $fr->Entry(-textvar => \$mac_addr)->pack(-side => 'left');
my $bt = $fr->Button(-text => 'Validate', -background => 'green');
$bt->configure(-command => sub { validate() });
$bt->pack(-side => 'left');
MainLoop;

sub validate {
    my $valid = '[0-9a-f]' x 2;
    if ($mac_addr =~ /($valid:){5}$valid/i) {
        print "Address '$mac_addr' is VALID\n";
    } else {
        print "Address '$mac_addr' is INVALID\n";
    }
}