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;
}