It was surprising to me that your system doesn't show any apparent floating point error. This prompted me to (finally) try the OP's the code myself, and sure enough, I had perfect rounding, too! Yet the OP's output corresponds to what I would actually expect from a double precision IEEE-754 float:

perl -le 'printf "%.18f", 10.1'
10.099999999999999645
[download]

Suspecting skulduggery, I dug into the Spreadsheet::ParseExcel version 0.65 code, and I was not disappointed. See Utility.pm, around line 800. Indeed, the module does its own rounding because of RT#45626:

    # Fix for Perl and sprintf not rounding up like Excel.
    # http://rt.cpan.org/Public/Bug/Display.html?id=45626
    if ( $data =~ /^${number_result}5/ ) {
        $number_result =
            sprintf "%0${str_length}.${after_decimal}f",
            $data . '1';
    }
[download]

This section was last changed ten years ago, suggesting OP may have a very old version, so your request for the version number was apropos.

Aside: The sub that's in is called ExcelFmt and is almost 900 lines long. The entire package is 45000 lines. I'm pretty sure the section around line 800 is responsible for the rounding, but that's a lot of code to grok in five minutes. I'd be happy to be corrected on the specifics, but the point is, something is doing the rounding. :-)

And yes, OP, I'd still suggest doing your own rounding for the reasons stated in my comment, to ensure you are rounding to your own specifications.