Find out what compiler/version was used to build the current perl, and recompile your app with it. g++ 3 is considerably fussier than v2 was about correct C++, particularly wrt namespaces. Most of the needed corrections are not too complicated; you may be able to get perl to do some of the editing for you!

It might be worthwhile to split the perl-calling functions off into a dll of their own. That would likely be the only thing needing recompilation with a change of perl.