use Net::Telnet::Cisco;

for ($i = 1; $i <= 254; $i++){
$device = "172.16.$i.251";

  my $session = Net::Telnet::Cisco->new(Host => "$device",
			Input_log => "perllogrouter/router$i.log",);

   $session->login('admin', 'admin');
   #some command
   $session->cmd ("show running-config ");
   $session->close;