http://search.cpan.org/~gaas/libwww-perl-6.02/lib/LWP/UserAgent.pm

##</code><code>##

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

my $base = 'http://www.uniprot.org';
my $tool = 'mapping';

my $params = {
  from => 'ACC',
  to => 'P_REFSEQ_AC',
  format => 'tab',
  query => 'P13368 P20806 Q9UM73 P97793 Q17192'
};

my $agent = LWP::UserAgent->new;
push @{$agent->requests_redirectable}, 'POST';

my $response = $agent->post("$base/$tool/", $params);

while (my $wait = $response->header('Retry-After')) {
  print STDERR "Waiting ($wait)...\n";
  sleep $wait;
  $response = $agent->get($response->base);
}

$response->is_success ?
  print $response->content :
  die 'Failed, got ' . $response->status_line . 
    ' for ' . $response->request->uri . "\n";