When called on a hash in list context, returns a 2-element list consisting of the key and value for the next element of a hash. In Perl 5.12 and later only, it will also return the index and value for the next element of an array so that you can iterate over it; older Perls consider this a syntax error. When called in scalar context, returns only the key (not the value) in a hash, or the index in an array.
Hash entries are returned in an apparently random order. The actual random order is specific to a given hash; the exact same series of operations on two hashes may result in a different order for each hash. Any insertion into the hash may change the order, as will any deletion, with the exception that the most recent key returned by each or keys may be deleted without changing the order. So long as a given hash is unmodified you may rely on keys, values and each to repeatedly return the same order as each other. See Algorithmic Complexity Attacks in perlsec for details on why hash order is randomized. Aside from the guarantees provided here the exact details of Perl's hash algorithm and the hash traversal order are subject to change in any release of Perl.
After each has returned all entries from the hash or array, the next call to each returns the empty list in list context and undef in scalar context; the next call following that one restarts iteration. Each hash or array has its own internal iterator, accessed by each, keys, and values. The iterator is implicitly reset when each has reached the end as just described; it can be explicitly reset by calling keys or values on the hash or array. If you add or delete a hash's elements while iterating over it, the effect on the iterator is unspecified; for example, entries may be skipped or duplicated--so don't do that. Exception: It is always safe to delete the item most recently returned by each, so the following code works properly:
Tied hashes may have a different ordering behaviour to perl's hash implementation.
This prints out your environment like the printenv(1) program, but in a different order:
Starting with Perl 5.14, an experimental feature allowed each to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24.
To avoid confusing would-be users of your code who are running earlier versions of Perl with mysterious syntax errors, put this sort of thing at the top of your file to signal that your code will work only on Perls of a recent vintage: