Hello doubledecker,

Just for the record: Spreadsheet::XLSX uses Spreadsheet::XLSX::Fmt2007, which in turn uses Spreadsheet::XLSX::Utility2007. The latter’s source code (lines 854–7, version 0.13) contains this comment:

### int2col
# convert a column number into column letters
# @note this is quite a brute force coarse method
#   does not manage values over 701 (ZZ)
[download]

Hope that helps,