#!perl
use strict;
use warnings;
use WWW::Mechanize;
use HTTP::Request;


my $ua = WWW::Mechanize->new();
my $r  = HTTP::Request->new(
    'POST' => 'https://api.blockchain.info/v2/receive/balance_update',
    [
        'Accept'         => '*/*',
        'Host'           => 'api.blockchain.info:443',
        'User-Agent'     => 'curl/7.55.1',
        'Content-Length' => '169',
        'Content-Type'   => 'text/plain',
    ],
'{"key":"[your-key-here]","addr":"183qrMGHzMstARRh2rVoRepAd919sGgMHb","callback":"https://mystore.com?invoice_id=123","onNotification":"KEEP", "op":"RECEIVE", "confs": 5}'
);
my $res = $ua->request( $r, );


__END__

Created from curl command line

curl -H "Content-Type: text/plain" --data '{"key":"[your-key-here]","addr":"183qrMGHzMstARRh2rVoRepAd919sGgMHb","callback":"https://mystore.com?invoice_id=123","onNotification":"KEEP", "op":"RECEIVE", "confs": 5}' https://api.blockchain.info/v2/receive/balance_update