Unfortunately, there is no central place for such situations. Newer version of modules usually describe their backwards compatibilty vars and funcs in their pod, as in
CGI, where it descibes interoperability with cgi-lib.pl, or
DBI, where certain variables have become simply meaningless. I guess the only solution then is to scan the pod of the module you plan to use and take the author's advice- most likely he knows best what he had in mind when he wrote the module.