This behavior isn't that suprising if you remember that each isn't returning you an iterator which is magically affected by the operations on previously used iterators -- or can magically affect subsequent iterators.

each is advancing the existing iterator contained in the internals of the hash you are working with -- that iterator is shared by everyone who has a refrence to that hash, in the same way the data inside the hash is shared by everyone with a refrence to it.

Put another way: If perl looked more like java, then i'd agree with you that this is a bug...

my $iter = %data.iter;
while (my ($key, $val) = each($iter)) {
   last if ($key eq 'drop_out');
   print("$key => $val\n");
}
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...but as it stands, this behavior is no more suprising to me then if you were looping over the keys to a hash, and modifying the values -- and then outside of the loop you discovered that the hash had been changed.