I have implemented it recently using HTTP::Server::Simple. I think the process should be similar on Dancer2.

(Also your server's IPN URL must use HTTPS and must not contain an IP address or a port number.)

The code is something like this:

my $USE_SIMULATOR = 0; # 0 = live, 1 = sandbox / simulator
my $LIVE_URL = 'https://www.paypal.com/cgi-bin/webscr'; # live
my $SANDBOX_URL = 'https://www.sandbox.paypal.com/cgi-bin/webscr'; # s
+andbox
my $PAYPAL_HOST_HEADER = ($USE_SIMULATOR ? 'www.sandbox.paypal.com' : 
+'www.paypal.com');
my $RESPONSE_URL = ($USE_SIMULATOR ? $SANDBOX_URL : $LIVE_URL);


# read POST params
my $postdata = read_all ($content_length, $server->stdin_handle);
my $post = (length ($postdata) > 0) ? CGI->new ($postdata)->Vars : {};


# extract some transaction details
my $txn_type = $post->{txn_type};
my $txn_id = $post->{txn_id};
my $payment_status = lc ($post->{payment_status});


# respond to Paypal's initial HTTP request
print "HTTP/1.1 200 OK\r\n\r\n";


# send back the original POST data with our extra field "cmd=_notify-v
+alidate" to get an INVALID or VERIFIED response
# this makes a new HTTP request
my $ua = LWP::UserAgent->new (ssl_opts => { verify_hostname => 1 });
my $req = HTTP::Request->new ('POST', $RESPONSE_URL);
$req->content_type('application/x-www-form-urlencoded');
$req->header(Host => $PAYPAL_HOST_HEADER);
$postdata = 'cmd=_notify-validate&' . $postdata;
$req->content($postdata);
my $res = $ua->request($req);


# process the response
if ($res->is_error)
{
    # connection error ...
}
elsif ($res->content eq 'VERIFIED')
{
    # ok ... ( further processing )
}
elsif ($res->content eq 'INVALID')
{
    # error ...
}
else
{
    # unexpected error ...
}
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