No, 0 and undef are not always the same. Take 'fork' as an example:

Does a fork(2) system call to create a new process running the same program at the same point. It returns the child pid to the parent process, 0 to the child process, or undef if the fork is unsuccessful.

So 0 indicates it is a child process while undef indicates failure. Not the same.

In general, you should be checking for definedness if your data is coming from outside of the script anyway. I usually do something like 'if( defined $foo && $foo ne ""){}' That way I avoid warnings about doing a string comparison with an undefined value (or whatever the warning message says). You are using 'strict' and 'warnings', right?

Anyway, your code above looks good.