It is not a matter of opposing standards. There are two different things. First, there are the function calls, whose behaviour is strictly defined (or else they would be unusuable). At the C level, failure is often indicated by a return value of -1, or NULL, and the reason of failure is passed in a global variable, errno. Not very intuitive (IMO), and Perl, being the thin layer it is above Unix/C, only makes it slightly more convenient. It returns 0 on failure. The reason of failure is still passed in a global variable, $! (which explains that variable).

The return values of spawned process have nothing to do with the shell. It's the other way around. Processes typically return 0 indicating success, and anything else for failure. There's no global variable to set, all they can do is return a single integer. "0 on success" is a mere convention. It's just because this convention is followed so often, typical shell Boolean logic is "reversed"; 0 is true, other integers are false.

-- Abigail