Practically every example in the demo, widget, does something like this, so do it like the examples.

TK textvariable not updating, Tk with map - Perl newbie

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

my $mw = MainWindow->new;

$mw->geometry('300x200');


my $timer; # global for timer

my @color = qw/red black/;
my $bits = pack("b16"x8,
"1111111111111111",
".11111111111111.",
"..111111111111..",
"...1111111111...",
"....11111111....",
".....11111......",
"......111.......",
".......1........",);

$mw->DefineBitmap('indicator' => 16,8, $bits);

my $label = $mw->Label(
        -bitmap=>'indicator', 
    -bg=>'black',
    -fg=>'red')->pack;

my $button = $mw->Button(-text => 'Start',
                 -command => \&change_label)->pack();


MainLoop;

sub change_label {   
   if ($button->cget('-text') eq "Start"){
         $button->configure(-text => 'Stop');
         $timer =  $mw->repeat(100,
                           sub{$label->configure(-fg=>$color[0]);
                               @color=reverse(@color);
                               });
         
         
         } else {        
               $button->configure(-text => 'Start');
               $timer->cancel;
             }

}
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