Works fine. Thanks. When unselecting the only selection of the multiple choice list, ncurses will return an empty array that cannot go through, so I've added a condition:

# (inside ncurses listbox callback)

my $listbox = shift;
my @sel = $listbox->id;

$byte2 = 0;
$byte1 = 0;

if (@sel) {
  my $code;
  $code |= 1 << $_ for @sel;
  $byte2 = $code >> 8;
  $byte1 = $code & 0xFF;
}
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I've taken that solution although the unpack and pack is interesting. Thanks again !

Cheers.

Al