use strict;
use warnings;
use LWP::UserAgent;

my $ua = LWP::UserAgent->new(keep_alive => 1);
my $url = "http://example.com/myUrl";
my $token = undef;

while(1)
{
	my $res = $ua->get($url, 'Start-From' => $token // "", 'X-AcceptDeltaUpdates' => 'true');
	$token = $res->headers->{'token'} if ($res->headers->{'token'});
	print $res->decoded_content if ($res->is_success);
	sleep 4;
}

##</code><code>##

Array is: ['item1','item2']
Add ['item3']
No update
Add ['item4']
No update
Add ['item5']
No update
...

##</code><code>##

use strict;
use warnings;
use AnyEvent;
use AnyEvent::HTTP;
my $url = "http://example.com/myUrl";
my $token = undef;

while(1)
{
	my $cv = AnyEvent::condvar;
	my $instance = http_request GET => $url,
        headers => { 'Start-From' => $token // "", 'X-AcceptDeltaUpdates' => 'true' },
        keepalive => 1,
        persistent => 1,
        sub {
			my ($data, $headers) = @_;
			$token = $headers->{'token'} if (defined $headers->{'token'});
			print $data if ($data);
			$cv->send;
		};
	$cv->recv;
	
	$cv = AnyEvent::condvar;
    my $timer = AnyEvent->timer(after=>4, interval => 4, cb => sub {
        $cv->send;
    });
    $cv->recv;
}

##</code><code>##

Array is: ['item1','item2']
No update
No update
No update
No update
No update
No update
Array is: ['item1','item2','item3','item4']
No update
No update
No update
No update
No update
No update
...