here is monkeypatching proof of concept, use bbox to get coordinates of the listitem, math, compare to mouse coordinates 
#!/usr/bin/perl --
$Devel::Trace::TRACE = 0; ## off
use strict; use warnings;

use Tk;
my $mw = MainWindow ->new ;

my $listFrame  = $mw->Frame->pack;


my $listBox = $listFrame->Scrolled(
                        'Listbox',
                        -scrollbars        => 'osoe',
                        -selectmode        => 'extended', 
                        -background        => 'white',
                        -height            => 10, 
                        -width             => 10, 
                        -selectborderwidth => 0,
                        -relief            => 'flat', )->pack;


$listBox->insert('end', qw/red yellow green blue grey/);

@ARGV and eval { require Tk::WidgetDump; $mw->WidgetDump; };
MainLoop;
exit 0;

# BeginSelect --
#
# This procedure is typically invoked on button-1 presses. It begins
# the process of making a selection in the listbox. Its exact behavior
# depends on the selection mode currently in effect for the listbox;
# see the Motif documentation for details.
#
# Arguments:
# w - The listbox widget.
# el - The element for the selection operation (typically the
# one under the pointer). Must be in numerical form.

sub Tk::Listbox::BeginSelect {
    package Tk::Listbox;
    use vars qw/ @Selection $Prev /;
    $Devel::Trace::TRACE = 1;    ## on
    my $w  = shift;
    my $el = shift;
    if ( $w->cget('-selectmode') eq 'multiple' ) {
        if ( $w->selectionIncludes($el) ) {
            $w->selectionClear($el);
        }
        else {
            $w->selectionSet($el);
        }
    }
    else {
        ## fix it here
        my ( $iltx, $ilty, $iw, $ih ) = $w->bbox($el);
        my $rooty = $listBox->rooty;
        my $evY   = $Tk::event->Y;
        my $ylt   = $rooty + $ilty + $ih;
        print "$ylt < $evY\n";
        if ( $ylt < $evY ) {
            ### clear the board when clicking off the end
            ### no selection 
            ### anchor
            $w->selectionClear( 0, 'end' )
        } else { ## the original logic
            $w->selectionClear( 0, 'end' );
            $w->selectionSet($el);
            $w->selectionAnchor($el);
        }
        @Selection = ();
        $Prev      = $el;
    }
    $w->focus if ( $w->cget('-takefocus') );
    $w->eventGenerate("<<ListboxSelect>>");
    $Devel::Trace::TRACE = 0;    ## off
}

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In reply to Re^2: How to determine if nothing is selected in a listbox (only select actually clicked element) with bbox by Anonymous Monk
in thread How to determine if nothing is selected in a listbox by simonz

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