When you say "the command fails", do you mean, nothing happens? Does the die statement from the open mention anything? Nothing in any of the special error variables?

No, the command line is executed; the command fails with an internal error. I've done more checking and found the problem is related to tieing the file that's being modified. Unfortunately, the untie command gives me an error: untie attempted while 1 inner references still exist. Since in my toy test script, all I do with File::Tie is tie a file to the array and immediately untie it, I'm slightly confused. Still.