use strict;
use warnings;

use LWP::UserAgent;

my $url = "http://127.0.0.1";
my $ctr = 0;

my $ua = LWP::UserAgent->new();
$ua->agent('BOGUS');

$ua->add_handler( request_preprepare => sub { $ua->agent("TEST $ctr"); print "User-Agent: ", $ua->agent, "\n"; $ctr++; });

my $response = $ua->get($url);