I assume that you are thinking about having the perl script exec'ed from a login script. Which is inherently insecure. A quick fingered user could hit ctrl-c just b4 the perl script was executed and get to the command prompt. I have actually done this personally. The best way to do this is to NOT have a real shell. ie set the users shell (in /etc/passwd) to the perl script (and add the script to /etc/shells on most systems). That way there NEVER is a functional shell for the possibly malicious user to get into.