If path is not the issue then permissions will be, either way if you were logging the error it would make debugging easier. As it happens scheduled tasks maintains a log file (Schedlgu.txt), in the c:\Windows folder. You can view the log from the Scheduled Tasks window by clicking View Log on the Advanced menu. You did look in there didn't you?

I suspect it fails to work because your script can not find "output.txt" (note you don't need to escape . in strings). The reason we don't know this is becuase you should be doing something like:

unless(open(OUTPUT,">$fname")) {
    log_error("Could not open $fname because $!\n");
    exit
}
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If you just exit silently you never know what happened. How you implement log_error is up to you. Writing to a file is generally easy and reliable although if you get a permission denied error on opening that how do you log that!