Yet another stupid mistake #1

During a refactor of a data from array @foo to hash %foo, I used 'each' to iterate over the hash, but forgot to change the 'for' statement with 'while'. So I ended up with something like:

$ perl -MData::Dump -E'%a=(a=>1, b=>2);
for (my ($k, $v) = each %a) { $_ = "$k x"; dd {k=>$k, v=>$v, "\$_"=>$_} }'

And this is nasty because for(@ary) aliases $_ to each element in @ary, and in this case it modifies $k (quiz #1: and $v too, do you know why?) right under your nose! Thus the result are really messed up:

{ "\$_" => "a x", "k" => "a x", "v" => 1 }
{ "\$_" => "a x x", "k" => "a x", "v" => "a x x" }

Not to mention the loop stops after processing two items (quiz #2: do you know why?) But you might not realize that after you add some pairs to %a and wondering why they don't get processed.

The error message Perl gives is not really helpful, to say the least :)