You can't post references; you presumably need to serialize them in some way; what is the server expecting?
From the error it looks like either the entire array or possibly each of the hashes in it should be serialized in JSON:
use JSON; # preferably have JSON::XS installed
my %prepared_conf = %$conf;
for my $field ( 'parameters', 'alerts' ) {
$prepared_conf{$field} =
JSON::to_json( $prepared_conf->{$field}, { 'ascii' => 1 } );
}
my $req = HTTP::Request::Common::POST($url,'content' => \%prepared_conf);