Now A and B submit the form at the same instant, does the variable $params->{key} hold the right value of each user? Is it possible for user B's submission to get the value of A's key?

No. Since you are not getting the value of $params->{key} from the memory of the server on which your application is running, but instead, your user is passing the value back to the server, every user is passing a unique, user-specific value. In other words... your script.cgi creates a random value of the key, sends it to user A in a form and user A sends that value back to script. Similarly, the script.cgi creates another random value of the key, sends it to the user B, who sends that value back to the script. Both users get different values. There is no clash whatsoever.

Read up on sessions, and everything will be clear.

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