If you are stuck consider another route, something like this

#!perl
use strict;
use WWW::Mechanize;

my $url = "http://www.example.com";
my $field_name = 'ctl00$ContentPlaceHolderContent$FileUpload_opfFile';
my $file_name  = 'c:/test/9780547076034NIMAS.opf';
 
my $mech = WWW::Mechanize->new();
$mech->get($url);
$mech->submit_form(
  form_number => 1,
  fields => { $field_name => $file_name },
);
die unless ($mech->success);
[download]

You may need to change the form number or change to form_name