The answer to your speculation " For some reason i get the below error" is most likely in the "input.log" file that tracks the session.

One big problem with your code is that it does no error checking.

You should check for errors when you "open DEVICE", and check for a valid $session when it is opened.

Most importantly, you should check for a non-zero return value from $session->login, and bail, if that fails.

open (my $DEVICE, "<", 'device.list') or die "Could not open device li
+st:$!";
while (<$DEVICE>) {
  chomp;
  my $routerIP = $_;
  print "Router IP is $routerIP,";
  my $session = Net::Telnet::Cisco->new(Host => "$routerIP", Input_log
+=>"input.log", 
               );
 if (! $session){
   warn "Could not open session to $routerIP\n";
   next;
 }
 if (! $session->login("$username", "$password") ){
      warn "Could not login to $routerIP\n";
   next;
 }
 my @output = $session->cmd("$cmd");

....  process ...
}
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