use strict;
use warnings;
use feature 'say';
use CAM::PDF;

sub _n { CAM::PDF::Node-> new( @_ )}
sub _p { CAM::PDF-> parseAny( \$_[ 0 ])}

my $doc = CAM::PDF-> new( 'test.pdf' ) or die;

# let's create a ListBox dictionary; its reference to be
# added to 2 arrays

my $forms_node = $doc-> getRootDict-> { AcroForm } or die;
my $forms_dict = $doc-> getValue( $forms_node );
my $fields_ary = $doc-> getValue( $forms_dict-> { Fields });

my $page_annots_node = $doc-> getPage( 1 )-> { Annots } or die;
my $page_annots_ary  = $doc-> getValue( $page_annots_node );

# it's funny how we code, below, half in Perl, half in kind of 
# DSL, i.e. write PDF syntax directly and make CAM::PDF to
# parse it into whatever it thinks appropriate. Said "PDF 
# syntax" is not too difficult and actually quite fun.

my $ref = _n( reference => $doc-> appendObject( undef, _n( 

# "1", below, is for "hidden";
# because of that, Rect is any, and Appearance is not even 
# dealt with here.
# "Ch" is "choice field", "list box"

    object => _p( q(<<
        /Type      /Annot
        /Subtype   /Widget
        /F         /1           
        /Rect      [ 0 0 1 1 ]  
        /FT        /Ch          
        /Opt       [
                        (My sneaky item #1)
                        (Item I'm very proud of)
                        (Hidden item nobody will find)
                   ]
        /T         (MyTopSecretListBox)
    >>))
), 0 ));

push @$fields_ary, $ref;
push @$page_annots_ary, $ref;

$doc-> cleanoutput( 'new.pdf' );

# let's re-open the modified PDF and test it
# by using buit-in CAM::PDF facilities to assign value
# to the added form field

$doc = CAM::PDF-> new( 'new.pdf' );

say for $doc-> getFormFieldList;

$doc-> fillFormFields( 
    MyTopSecretListBox => << 'END_OF_IT'
Hidden item nobody will find
But we are not limited to our silly list, if value is 
set programmatically
END_OF_IT
);

$doc-> cleanoutput( 'new2.pdf' );

##</code><code>##

$ perl acroform.pl 
Text Box 1
MyTopSecretListBox

$ pdftk test.pdf dump_data_fields
---
FieldType: Text
FieldName: Text Box 1
FieldFlags: 0
FieldValue: 
FieldJustification: Left

$ pdftk new.pdf dump_data_fields
---
FieldType: Text
FieldName: Text Box 1
FieldFlags: 0
FieldValue: 
FieldJustification: Left
---
FieldType: Choice
FieldName: MyTopSecretListBox
FieldFlags: 0
FieldJustification: Left
FieldStateOption: Hidden item nobody will find
FieldStateOption: Item I'm very proud of
FieldStateOption: My sneaky item #1

$ pdftk new2.pdf dump_data_fields
---
FieldType: Text
FieldName: Text Box 1
FieldFlags: 0
FieldValue: 
FieldJustification: Left
---
FieldType: Choice
FieldName: MyTopSecretListBox
FieldFlags: 0
FieldValue: Hidden item nobody will find&#10;But we are not limited to our silly list,&#10;if value is set programmatically#10;
FieldJustification: Left
FieldStateOption: Hidden item nobody will find
FieldStateOption: Item I'm very proud of
FieldStateOption: My sneaky item #1