The problem is that the Text widget has default bindings to a Popup menu when you right click. You can either make a custom derived Text widget to remove the Popup menu, and replace it with your code, or you can let the Popup do the work. See Making a derived Tk::Text object for how to make a custom derived Tk::Text widget.

To let the Popup do the work, try something like this:

#!/usr/bin/perl

use Tk;

my $mw = MainWindow->new;
my $txt = $mw->Text()->pack(-side => 'top');

my $number = 1;
my $letter = 'a';
my $popup_menu = "";

my $new_menu = $mw->Menu();

$new_menu->add('command', 
           -label => "Click me to show the pop-up menu.", 
           -command => sub { 
           $popup_menu = $mw->Menu();
           $popup_menu->add('command',
                    -label => "Number: $number",
                    -command => sub {
                    print STDERR "Number: $number\n";
                    $number++;
                    }
                    );
           $popup_menu->add('command',
                    -label => "Letter: $letter",
                    -command => sub {
                    print STDERR "Letter: $letter\n";
                    $letter++;
                    }
                    );
           $popup_menu->Popup(qw/-popover cursor/);
           }
           );

$txt->menu( $new_menu );

MainLoop;
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