use Errno qw/EAGAIN EINTR/ ;

$result = $sem->op(0, -1, SEM_UNDO | IPC_NOWAIT)
if( !$result && $! == EAGAIN ) {
   # semaphore was zero, no slots available
   print "BUSY, try later\n" ;
   exit(0) ; 
}
die "op failed: $!" unless $result ; # op failed for some other reason

##
## lock acquired
## 


##</code><code>##

alarm(5) ; # five seconds
$success = semop
$result = int $! ; # capture the value of errno 
alarm(0) ; # cancel alarm(if still active)
if( !$sucess && $result == EINTR ) {
   # timed out
}