It looks like your Perl was compiled to use "long double" for floating point numbers, hence 18 digits of precision instead of 15 (i.e. "%.18g" format) for default output. But Excel stores numbers as IEEE-754 doubles, so you simply loose precision on export to xls. What you are reading back from xls is worse approximation of e.g. "10.1" (which can't be represented precisely in binary floating point of whatever size) from your Perl POV, than what it converts source string "10.1" to.
Not sure if my next advice will be popular, but if your task is relatively short/simple scripts, and if you have to compare floating point numbers restored from xls binary data with e.g. user input (or any ASCII text file input), then, for simplicity/sanity, it just could be easier to use Perl which uses plain old IEE-754 doubles.
>perl -V:nvsize
nvsize='8';
>perl -MConfig -E "say $Config{uselongdouble}"
>perl -E "say unpack 'd', pack 'd', 10.1"
10.1
>perl -E "say unpack 'f', pack 'f', 10.1"
10.1000003814697
