Tk-JComboBox 1.12 has been uploaded to CPAN. I think it will resolve this issue. This release also includes minor updates to the docs and test cases. It also features a new examples directory that should grow larger over time. Here is a relevant example that demonstrates -popupcreate

use Tk;
use Tk::JComboBox;

my $mw = MainWindow->new;
my $entry = $mw->Entry->pack;

my $jcb = $mw->JComboBox(
   -choices => [qw/one two three four/],
   -entrywidth => '16',
   -highlightthickness => 0,
   -listwidth => '16',
   -mode => 'readonly',
   -popupcreate => \&addItems,
)->pack;

MainLoop;

sub addItems {
   my @items = split(/ /, $entry->get());
   $jcb->removeAllItems;
   $jcb->configure(-choices => \@items) if @items;
}
[download]

For versions 1.11 and earlier the following serves as a partial workaround to the issue with -popupcreate. The last line can be included in callbacks to take care of Popup layout/positioning issues.

sub addItems {
   my @items = split(/ /, $entry->get());
   $jcb->removeAllItems;
   $jcb->configure(-choices => \@items) if @items;
   $jcb->PopupCreate;
}
[download]

Note that this will still have a problem if the JComboBox is empty, because the popupcreate callback will not get triggered. Version 1.12 accounts for this.

Rob