>>>    (let* ((gh   (assoc-ref %build-inputs "source"))
>>
>> What is the associated value here?  I assume it should be 'gh.pl'.

> Yes.  Actually, “source” is the name of the input that corresponds to
> the ‘source’ field of the package.  So, if your package does

>   (package
>     (source (origin (... (uri ".../gh.pl"))))
>     ...)

> then in the builder, (assoc-ref %build-inputs "source") will be
> /nix/store/...-gh.pl.

Makes sense, but I don't understand the (uri ".../gh.pl") part.  It
should be (uri "...tar.gz") instead.