for ( ... ; ... ; ... ) is not a counting loop. It never creates a counter. It's really a while loop in disguise (in terms of function).

What's the process's exit code? It will tell you which signal killed the process if it was a signal (e.g. SIGSEGV, SIGCHLD, SIGPIPE), and the exit code will likely be an errno if the process didn't die from a signal. (perl -E'say $!=123;' to find out what errno 123 means.)