How about packing the response into a variable and changing it after the first time?

my $response = "NB_CLT_7.6.0.2\r";
...
...
[ qr/Enter pack name \(or q\) \[q\]:\s+/ => sub {
                 $exp->send($response);
                 $response = "q\r";
                 exp_continue;
                 }
         ],
[download]

Admittedly, I have never used Expect so I cannot test whether this would work. Also, this would involve a closure of $response, so I am not sure...