If you don't feel like patching Tk (probably somewhere here), you can check for the selection periodically using a timer:

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

my $mw = 'MainWindow'->new;
my $lbox = $mw->Listbox->pack;

my @list = qw( a b c d e f );
$lbox->insert(end => @list);
$lbox->selectionSet(1);

my @selection;
$lbox->repeat(100, sub {
    my @current = $lbox->curselection;
    warn "Listbox changed!\n" if "@selection" ne "@current";
    @selection = @current;
});

my $f = $mw->Frame->pack;
$f->Button(-text    => 'Add',
           -command => sub { $lbox->selectionSet(2) })->pack(-side => 
+'left');
$f->Button(-text    => 'Clear',
           -command => sub { $lbox->selectionClear(0, 'end') })->pack;

MainLoop();
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