This backwards compatibility is a conscious decision. And it is argued about. To find a bit to read about that, Google e.g. for darkpan. (It is probably one reason why Perl 5 is so extensible.)
"There is currently some FUD going about in some parts of the Perl community about why we should break Perl 5 backwards compatibility. A short blog entry, schmarkpan, is a good example of the trend: loud, noisy, but clueless and devoid of any content."
