On *nix, the value returned by system doesn't necessarily reflect an OS error

Which is a common newbie error. They check the value of $! inappropriately after system. According to perlvar, $! (or $^E on some systems) will have useful information after system or backticks if (and only if) $? == -1. Hence the check.

What distinct information is available on exit from a VMS, Win32 or OS/2 system call? How much different are the extended error returns?

system returns the same. However, system may fail with $? = -1 when the child process fails due to an OS error. $! is C's errno, but on VMS, Win32, and OS/2 the underlying API provides it's own error messages which C's errno does not reflect. On these systems $^E holds that value (see perlvar for the specific API calls made to get that value). This is a workaround for the fact that Perl and C are still a bit *nix centric. $^E exists for people who aren't on *nix systems while still allowing *nix programmers on Windows or the like still get what they expect from $!.

Cheers,
Erik

Light a man a fire, he's warm for a day. Catch a man on fire, and he's warm for the rest of his life. - Terry Pratchet