I use Expect enough but via ssh, not exactly sure about serial port usage. I would assume from ancient experience that you would use setserial to configure the port and then get a r/w IO::Handle for /dev/ttyS0 or whatever and pass that to Expect->exp_init($fh). Or maybe spawn a kermit and interact with that to handle all of the serial setting up stuff. After you have an Expect object, it's pretty simple, make sure to read the perldoc to the end, there are several examples there (that's where I picked up everything). Some useful things:

use Expect;
$Expect::Debug = 1;    # verbose debug output
$Expect::Log_Stdout = 1;  # show chatter for debugging

# stolen from Net::Telnet::Cisco I think.
my $prompt = '[\w-]+\s?(?:\(config[^\)]*\))?\s?[\$#>]\s?(?:\(enable\))
+?\s*$';

my $exp = Expect->spawn( @command );
# or
my $exp = Expect->exp_init($filehandle);

sub cmd { my ($exp, $cmd) = @_;
 $exp->print($cmd, "\n");
 $exp->expect($timeout, -re => $prompt) or return;
 my $output = $exp->before;
 $output =~ s/\r//g;  # silly telnet line endings
 $output =~ s/^$cmd\n//;  # remove the sent cmd
 return $output;
}

sub login { my ($exp, $user, $pass) = @_;
 my $rc = $exp->expect($timeout,
  [qr/[Pp]assword:\s*/, sub { my $exp = shift;
   $exp->print($pass,"\n");
   exp_continue;
  }],
  [qr/[Uu]sername:\s*/, sub { my $exp = shift;
   $exp->print($user,"\n");
   exp_continue;
  }],
  -re => $prompt,
 );
 return defined($rc) ? $rc : 0;
}

sub enable { my ($exp, $pass) = @_;
 $exp->print("enable","\n");
 $exp->login('',$pass);
}

# generic chat once you have your $exp from a spawn or filehandle

login($exp, "username", "password");
cmd($exp,"terminal length 0");
enable($exp, "enablepass");
my $config = cmd($exp, "sho mem");
cmd($exp, "exit"); # enable
cmd($exp, "exit");
$exp->soft_close;
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But I really put those subs in a subclass of Expect so I can just call them as $exp->cmd('foo"). The expect function can match simple strings or regex, and there's the giving it an arrayref with a match and coderef to be executed on match and the exp_continue which causes it to restart the matching process. Mostly it's just the print and expect and before methods doing most of the work. I haven't needed to muck about with the pty/tty options in any of the router/switch tweaking scripts I've written so far.