In each release of Perl, perldelta will contain an overview of the changes since the previous major release. However, the older perldelta documents are also available, with the version inserted into the name of the document.

That is, perl5.004 comes with perldelta; perl5.005 comes with perldelta and perl5004delta; perl5.6 comes with perldelta, perl5005delta, and perl5004delta.

(The CHANGES files list all the changes between Perl versions, the big ones and the little ones, in the order they were made.)