Since you did catch any possible mkdir errors, these will, as you have it, be caught by the web server's error log. You can use the CGI::Carp with fatalsToBrowser to have the errors reported to the browser.

As for the error, the script will be running as the user that the web server is running under; if the directory that is indicated is in a directory that is not writable by that particularly user, mkdir will fail with write permissions. When you ran it from the command line, it uses your permissions, which are probably sufficient to complete the operation. In addition, you will be making the directory in the same location where the CGI script is being run, whcih is probably not your goal; you probably want to use absolute paths to point to exactly where you want the directory to be made.

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Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain
It's not what you know, but knowing how to find it if you don't know that's important