"If your program is not written to use threads, Perl won't use threads and all the concurrency problems you have come from other sources."

What does this mean? Indeed, my program "was not written to use threads" in that is was developed and tested on a single processor machine. Yet the same code fails on a multi-processor machine with threading enabled. The actual code is simple: I make a system call to a routine that outputs a file, then reads the resulting file. The perl script is trying to access the file before the called routine is finished making it (if I load the code in a debugger and step through line by line it runs just fine). How does this translate to "perl not using threads"? Are you suggesting that it's a problem with the OS managing threads?