It's noteworthy that one of the people behind this project, is Mark Adler, the man who originally created the zip algorithm for PKZIP, the (original) ZIP utility for DOS. The other one is Jean-loup Gailly, the author/maintainer of gzip. Therefore it's not surprising that zlib (and thus Compress::Zlib) can handle both gz and zip compression.

For the algorithm, there's indeed no better place to start than the zlib homepage. You will have to delve into the C code, as the code itself and its comments, are the actual main documentation.