Erm... not really anything to do with Perl is it?
The 'su' command changes user credentials to those of the root user or to the user specified (in your case 'tuser') and then initiates a new session - you basically become the specified user and can then execute commands as them.
The 'sudo' command allows you to execute commands as another user - this is not the same as above. In order to execute the sudo command your user needs to be defined in the sudoers file (typically stored at /etc/sudoers) - the error messae above is telling you that suser is not defined in this file and so you can't use sudo...