But my "error" file ends up with 8 million records in it! So I've got something I'm missing here and I was hoping another pair of eyes would catch it.

You're printing the wrong array (@data instead of @fields) to your errors file.

    # split the fields of each record
    my @fields = split(/,/, $line);

    # Check if the storeNbr field is empty.  If so, write record to er
+ror file.
    if (!length $fields[28]) {
        print $ERR_FH join (',', @$_), $/ for @data;
    }
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@data is where you're accumulating your good records; and every time you find a bad record, you're writing all the good records you've accumulated so far into your errors file, instead of the single bad record.