This is a generally useful package with some minor and easily-fixed issues; fixing these would make it very useful.
The package tries to use itself, so any code using it needs to explicitly require the base package Math::BigInt as well as Math::BigInt::Named.
There are a couple typos in the number names in the English version ("twelve" (twelf) and "forty" (fourty) were the ones I noticed in a glance at the source code).
Spacing is handled strangely in the English version ("onehundred" instead of "one hundred"; presumably the equivalent is correct in German).
The package uses the old-style British names for numbers >= 10^9 (milliard, etc.) Not a problem per se but probably not what most users would expect, and ideally a future version would split the English package to milliard and non-milliard versions.