I'm in a similar position, except with gcc instead of perl.

The project I'm working on is written in C++, and is compiled with gcc 3.0. I once tried to compile everything with gcc 3.4. I've got loads of compilation errors, because newer versions of gcc try to approximate the C++ standard more closely, especially with templates. I started to correct them. I wanted to achive that those modules of the project that I need would compile cleany with both versions of gcc. (There are lots of other modules.) I've got permission to do any corrections, as long as they don't break with the older gcc, and they are clearly marked in the CVS log.

I temporarily abandonned the whole proting thing when I realized that it's not only syntax issues that I'd have to change, but also the code depends on an internal class of the older version of libstdc++. The class is clearly marked internal and "may disappear in future versions", and its name starts with an underscore. I don't really understand what the class does, so I didn't dare touch it.

Since then, gcc 4.0.0 came out.