CGI::script_name expects the CGI environment variable SCRIPT_NAME to contain the name of the notional script being executed. Running in the context of a web server such as Apache the variable is set by the calling code. However if you are running the script in the context of a cron job then very likely SCRIPT_NAME is not being set. You could fix this by having the cron job run a bash script that sets SCRIPT_NAME before calling your Perl script.