I'm not really sure what the 'challenge' is? Do you want to be able to submit that form?

use strict;
use warnings;
use WWW::Mechanize;

#takes what vacancy to search as first argument on command line 
my $mech = WWW::Mechanize->new();

$mech->get("https://thebigword-careers.irecruittotal.com/cac/SearchVac
+ancy.aspx?EmploymentTypeID=0&Intranet=0");

my $vacancy = $ARGV[0];
$mech->field(
"ctl00$mvMintPP$ctl00$ContentPlaceHolder_Main$mvMintPP$ctl00$txbJobRef
+", $vacancy);
#(^^without plus sign occuring copied over)
$mech->click_button(value => "Search Vacancies");

$mech->dump_text;
[download]

..might work..