Facicons are for web pages, not applications. This relates to your application, rather than the packaging of the application.

use strict;
use warnings;

use Tk;
my $mw = MainWindow->new;
$mw->title("Icon test");

# comment out the next two lines to see the red Tk logo in the to left
my $xpm = $mw->Photo(-file => "testicon.xpm", -format => 'xpm');
$mw->Icon(-image => $xpm);    

$mw->Button(-text => "Done", -command => sub { exit })->pack;
MainLoop;
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