Thanks for the referenced documentation, moritz.
The algorithm described before the source is not clearly just a SHA-256 digest.
I was able to compile the source on *bsd , just providing auxiliary implementations for mempcpy() and stpncpy() GNU functions.
I preferred a pure Perl implementation just because my authentication scripts are all Perl, and for future portability. Before trying, I believed that the sha256-crypt from glibc2.7 was too much GNU to port it with ease.

Unfortunately so, I'm not going to write sha256-crypt for Perl, but I will follow moritz' suggestion and plug this external C binary : /