I can't think of an example where an old function was deprecated, because that would cause old code to break. Do you have an example of this?

See perldeprecation, and though some of the features may seem obscure, I do believe that they caused some breakages. As I recall, P5P did proceed quite carefully for each of them, checking breakages on CPAN etc. Also, they've generally fallen into two categories: One, things that have been deprecated for ages (e.g. $* and $# are documented as "Deprecated in Perl 5. Removed in Perl v5.10.0.", but weren't fatal until 5.30), or two, changes to make room for new syntax in the future (e.g. the notes for "Some formerly deprecated uses of an unescaped left brace "{" in regular expression patterns are now illegal" in 5.30 state "... will enable the Perl language to be extended in various ways in future releases."). Update: I guess a third category would be experiments that were removed - perlexperiment.