My response is embedded as comments in my revised version of your sample program:

#!/usr/bin/perl

use warnings;
use HTTP::Request::Common qw(GET POST);
use POE qw(Component::Client::HTTP);
use Time::HiRes qw(time);

# TODO - If @url_list gets very large, see the first example at
# http://poe.perl.org/?POE_Cookbook/Web_Client for known pitfalls and
# solutions.

my @url_list;
for (1 .. 10) {
  my $a   = rand(900);
  my $b   = rand(900);
  my $t   = int(rand(100000));
  my $url = "http://127.0.0.1:80/search/$t/?a=$a&b=$b";
  push @url_list, $url;
}

# TODO - POE::Component::Client::HTTP uses default keep-alive socket
# concurrency and timeouts that are friendly to web sites but may not
# be appropriate for benchmarking.  To change that, see the
# ConnectionManager at:
# http://search.cpan.org/~rcaputo/POE-Component-Client-HTTP-0.947/lib/
+POE/Component/Client/HTTP.pm#spawn

POE::Component::Client::HTTP->spawn(Alias => 'ua');

sub got_response {
  my $now = time();

  my ($SESSION, $heap, $request_packet, $response_packet) =
    @_[SESSION, HEAP, ARG0, ARG1];

  my $http_request  = $request_packet->[0];
  my $http_response = $response_packet->[0];

  # Find the time since the "tag" was defined.
  # This may not be appropriate for benchmarking since a lot of
  # interpreter-level work has been done to get here.  HTTP headers
  # have been parsed by HTTP::Response, for example.  This overhead
  # can't be separated from the web server's performance at this high
  # level.

  print "$http_request:\n", ($now - $request_packet->[1]), " sec\n";
}

sub _start {
  my $kernel = $_[KERNEL];

  foreach my $url (@url_list) {
    $kernel->post(
      "ua" => "request",
      "got_response",
      GET($url),
      time(),     # Called a "tag" in PoCo::Client::HTTP's docs.
    );
  }
}

POE::Session->create(package_states => [main => ["_start", "got_respon
+se"]]);

POE::Kernel->run();
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