Calls the System V IPC function shmctl. You'll probably have to say
use IPC::SysV;first to get the correct constant definitions. If CMD is IPC_STAT, then ARG must be a variable that will hold the returned shmid_ds structure. Returns like ioctl: undef for error; "0 but true" for zero; and the actual return value otherwise. See also "SysV IPC" in perlipc and the documentation for IPC::SysV.
Portability issues: "shmctl" in perlport.