Fcntl loads the constants defined in the system's <fcntl.h> C header file, and
used with both the fcntl(2) and open(2) POSIX system calls.
- To perform a fcntl(2) operation, use IO::fcntl.
- To perform an open(2) operation, use IO::sysopen.

The set of operations and constants available depends upon specific operating
system. Some values listed below may not be supported on your system.