Changes between 5.0 and 5.6

I was doing some scripting on my debian machine which has Perl 5.6.1 
installed, and I coded the following (which worked flawlessly):

foreach (%data){
    $_ =~ <something here>;
}

Now, when I accessed $data{$somekey} after this loop, I found the 
changes had been made as expected.
However, when I tried this on a FreeBSD box running 5.0, all changes 
made during the foreach loop were lost after it finished, and I had to 
replace the previous with this:

foreach (keys %data){
    $data{$_} =~ <something here>;
}

My question is, when did the change occur between 5.0 and 5.6?  I can't 
find anything on perl.org documenting the difference between past 
versions (which I will need if I continue to cross develop between 
versions).

Thank you.
John