This function writes to the shared memory segment ID starting at
POS for size
SIZE (by attaching to it, copying in, and
detaching from it). If
STRING is too long, only
bytes are used;
STRING is too short, nulls are written to fill out
function returns true if successful, or false if there is an error.
On error, it puts the error code into $!. This function is
available only on machines supporting System V IPC.