use v5.14;
use CGI ();
use JSON::PP ();

my $q     = 'CGI'->new;
my $json  = 'JSON::PP'->new;
my $input = $json->decode( $q->param('POSTDATA') );

my $a = $input->{a};
my $b = $input->{b};
my $c = $a + $b;

print $q->header("application/json");
print $json->encode({ c => $c });

##</code><code>##

use v5.14;
use HTTP::Tiny ();
use JSON::PP ();

my $json = 'JSON::PP'->new;
my $http = 'HTTP::Tiny'->new;
my $url  = 'http://buzzword.org.uk/2013/json-addition.cgi';

my $input    = { a => 40, b => 2 };
my @request  = ($url => { content => $json->encode($input) });
my $response = $http->post(@request);
my $output   = $json->decode($response->{content});
say $output->{c};