use IPC::SysV qw(IPC_PRIVATE S_IRWXU IPC_CREAT);
use IPC::Semaphore;

$sem = new IPC::Semaphore(IPC_PRIVATE, 10, S_IRWXU | IPC_CREAT);
# and the crucial part:

$sem->op($semaphoreNumber,0,0);
# where first zero means wait-for-zero operation,
# ie, your code will stop and wait (CPU-friendly) 
# until your semaphore becomes 0.
#etc...