Regarding the first question: you can first insert the items, then disable the list box. That way the items will be displayed:

use strict;
use warnings;
use Tk;
my $w = MainWindow->new();
my $l = $w->Listbox()->pack();

for (qw(foo bar baz)){
    $l->insert('end', $_);
}

# disable it afterwards

$l->configure(-state => 'disabled');

MainLoop;
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I don't see why $l->configure(-state => 'normal'); or $l->configure(-state => 'disabled'); shouldn't work for enabling/disabling the box. Try to insert a few prinit statements to verify that you're actually executing the code you think you are executing.