system returns the exit status of the command being run. Compare that against backticks (or the qx equivalent) which return the output of the command being run.

But, please note the contributions by my learned brethren exhorting you to greater care over validation and cleansing of your inputs. You are using taint mode, which is good, but you are using it on entirely the wrong script. post.cgi is the one where taint mode will have the greater effect.