However, sometimes, what is available is a little less than what is wanted.

Whenever you encounter a module which provides a little less, you have 3 ways to add the missing bits (using Number::Format and Util::Number in the following):

1. copy the source of Number::Format to Util/Number.pm, add the missing subroutine and use your renamed private copy (brittle)
2. write Util::Number as a subclass which uses Number::Format and extends its functionality with your own subs (better)
3. switch packages in your code and add the enhanced subroutines there (hack)

The first decouples you from the development of Number::Format. You'd have to update your private module if you want to benefit from the improvements of Number::Format as it developes.

The second possibility places on you the tiny burden of writing tests for your module, but only for those bits you have added.

The third may be used if and only if a) you use Number::Format only in one place, b) the added bits are trivial, c) you know where to look when things break.

So, I'd go with number 2.

package Util::Number;
use parent Number::Format;

sub pretty_number {
    ...
}
1;
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