snmpset  <router>  <RW Community String>  .1.3.6.1.4.1.9.2.1.55.<tftp server ip> string <filename>

##</code><code>##

  $where = "10.6.21.100 string $tftpfile";
  @param = qw($oid SET_REQUEST $where);
  ($session, $error) = Net::SNMP->session( -hostname  => $router
                                           -community => $community
                                         );      
  my $response = $session->set_request(@param);

##</code><code>##

$response = $session->set_request(@oid_type_value);

##</code><code>##

<tftp server ip> string <filename>