Every hash variable in Perl has its own internal iterator, accessed
by the builtin each, keys, and values functions. The iterator is
also implicitly used whenever the hash is evaluated in list context.
The iterator is "reset" whenever keys or values is called on a
hash, including the implicit calls when the hash is evaluated in
list context. That makes it dangerous to do certain hash operations
inside a while ... each loop.

Hash::SafeKeys provides alternate functions to access the keys,
values, or entire contents of a hash in a way that does not reset
the iterator, making them safe to use in such contexts.