Actually $? is not a boolean. As documented in perlvar, it is the return code of the last system call. That is 0 for success, and all else is an error. If the error is under 256, then it is a system error. If the error is over, then divide by 256 and you have the program's return code. The return code of your programs are set by exit, are 0 by default, and are set from $? on a die.

Unfortunately that means that every program has its own possible meanings for exit codes. Traditionally, however, 1 means "Error, but not an important one" and 2 means "Bad error". For instance rm on my system will return 1 if you don't give it any files to delete, but a 2 if you try to delete something and it can't.