Here's one implementation of Tk::after using a status var to indicate if the counter should be incremented

#!/usr/bin/perl

use Tk;

use strict;
use warnings;

my $count = 0;
my $count_on = 0;

my $window = MainWindow->new;
$window->title("My Example");
$window->Label(-textvariable => \$count )->pack;
$window->Button(
    -text => "Start",
    -command => sub {$count_on = 1},
    )->pack;
$window->Button(
    -text => "Stop",
    -command => sub {$count_on = 0},
    )->pack;

my $timer = Tk::After->new($window, 1000, 'repeat', sub { $count++ if 
+$count_on });

MainLoop;
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