You can use the size() method of the Listbox object to retrieve the number of elements in the Listbox. You can use this value for the -text option of the Label object or you can store it in a variable.

Assuming you don't have any need to store the number of items in the Listbox in a variable for other needs, the following code sample will dynamically update the Label widget as items are added and removed from the Listbox.

#!/usr/bin/perl

use strict;
use Tk;

my $mw = MainWindow->new(qw/-width 300 -height 200/);

my $lb = $mw->Listbox(-selectmode => "multiple")->pack();
my $label = $mw->Label(-text => "Empty")->pack();
my $add = $mw->Button(-text => "add item",
                      -command => \&add
                     )->pack();

my $delete = $mw->Button(-text => "delete item",
                         -command => \&delete,
                        )->pack();
MainLoop;

sub add {
    $lb->insert('end', "item");
    $label->configure(-text => "num elms = " . $lb->size());
}

sub delete {
    my @elms = $lb->curselection;
    foreach (@elms) {
        $lb->delete($_);
    }
    $label->configure(-text => "num elms = " . $lb->size());
}
exit;
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hope this helps,
davidj