For a simple case (where you are only allow a single item to be selected at a time, you could try something like this:

use Tk;
use strict;
use warnings;

my $value;

my $mw = MainWindow->new;

my $entry = $mw->Entry(
   -state => 'readonly',
   -textvariable => \$value
)->pack;

my $lb = $mw->Listbox(
   -height => 0
)->pack;
$lb->insert('end', qw/one two three four five six/);
$lb->bind('<ButtonPress-3>', [\&setItem, \$value, Ev('@')]);

sub setItem
{
   my ($lb, $valSR, $xy) = @_;

   $lb->selectionClear(0, 'end');
   my $index = $lb->index($xy);
   if (defined($index)) 
   {
      $lb->selectionSet($index);
      $$valSR = $lb->get($index);
   }
}

MainLoop;
[download]

A slight variation of this:

use Tk;
use strict;
use warnings;

my $value;

my $mw = MainWindow->new;

my $entry = $mw->Entry(
   -state => 'readonly',
   -textvariable => \$value
)->pack;

my $lb = $mw->Listbox(
   -height => 0
)->pack;
$lb->insert('end', qw/one two three four five six/);
$lb->bind('<ButtonPress-3>', [\&setItem, \$value]);

sub setItem
{
   my ($lb, $valSR) = @_;
   my $xy = $lb->XEvent->xy;
   $lb->selectionClear(0, 'end');
   my $index = $lb->index($xy);
   if (defined($index)) 
   {
      $lb->selectionSet($index);
      $$valSR = $lb->get($index);
   }
}

MainLoop;
[download]

Note the modification to the <Button-3> bind, and the first two lines of the setItem sub. I used to prefer the latter rather than the former, because I thought that it was easier to read, but I've since changed my mind -- especially when coding a Tk module, because I find the former is easier to unit-test reliably across different platforms. Your mileage may vary...