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
##