Except the file name itself (K9-RW), all directories exist.

I would strongly suspect this is not in fact the case - possibly due to subtle typos, or differences in case, or control characters or spaces. Try adding the following check on the line before the sysopen:

while ($outputFileName =~ /.*?\//g) {
    die "no dir: $`$&\n" unless -d "$`$&";
}
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Dave.