This

Content-Type: application/x-www-form-urlencoded

grant=client_credentials&partner={partner}&client={client}&secret={sec
+ret}
[download]

Makes me think it is not expecting json encoded body but a normal http format.

via https://stackoverflow.com/questions/3635288/perl-programatically-set-post-param-using-restclient-module try

my @body_list = (grant => 'client_credentials',
                      partner => ‘dfjkdjfdkdjkdkdjfdk’,
                      client =>  'dkfjdlfjdlfkjlj',
                      client => 'dfkljflsfjlajflafjlsdkfjdskl=',  # ne
+ed encoding that is = will be %3D
                    );     
my $bodyData=substr($restClient->buildQuery(\@body_list),1);
[download]

im willing to bet the substr from 1 is to get rid of a leading ?