OK, I've just found out the answer. Apparently, according to the perl 5.6 version perlfaq4: 

lwall In Perl 4, you were not allowed to modify a hash at all while
iterating over it.  In Perl 5 you can delete from it, but you still
can't add to it, because that might cause a doubling of the hash table,
in which half the entries get copied up to the new top half of the
table, at which point you've totally bamboozled the iterator code.
Even if the table doesn't double, there's no telling whether your new
entry will be inserted before or after the current iterator position.

Either treasure up your changes and make them after the iterator finishes,
or use keys to fetch all the old keys at once, and iterate over the list
of keys.
I guess perlmonk's docs must be from 5.005_3. I checked on perl.com as well beforehand, but they have the same answer. Does that mean that perl 5.6 isn't considered authoritative yet?

In reply to Re: Adding or removing keys while iterating over a hash by mischief
in thread Adding or removing keys while iterating over a hash by mischief

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