See the current Perl documentation for shmctl.
Here is our local, out-dated (pre-5.6) version:

shmctl - SysV shared memory operations

shmctl ID,CMD,ARG

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 which will hold the returned shmid_ds
structure. Returns like ioctl: the undefined value for error, ``0 but true'' for zero, or the actual return value otherwise. See also IPC::SysV documentation.