As far as I understand, backward compatibility with Perl5 (not Perl4) is an issue in develloping Perl6 and the RFC process. Those things that went into Perl5 for backward compatibility with Perl4 will surely disappear. The latest doc mentions which commands are depreciated. Also if you post code, the monks will tell you if you use such code. So by the time Perl6 will come, you should be able to use it with only minor changes here & there, as