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each HASH
each ARRAY
each EXPR

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 subject to change in future versions of Perl, but it is guaranteed to be in the same order as either the keys or values function would produce on the same (unmodified) hash. Since Perl 5.8.2 the ordering can be different even between different runs of Perl for security reasons (see "Algorithmic Complexity Attacks" in perlsec).

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, entries may be skipped or duplicated--so don't do that. Exception: In the current implementation, it is always safe to delete the item most recently returned by each(), so the following code works properly:

while (($key, $value) = each %hash) {
  print $key, "\n";
  delete $hash{$key};   # This is safe

This prints out your environment like the printenv(1) program, but in a different order:

while (($key,$value) = each %ENV) {
    print "$key=$value\n";

Starting with Perl 5.14, each can take a scalar EXPR, which must hold reference to an unblessed hash or array. The argument will be dereferenced automatically. This aspect of each is considered highly experimental. The exact behaviour may change in a future version of Perl.

while (($key,$value) = each $hashref) { ... }

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:

use 5.012;	# so keys/values/each work on arrays
use 5.014;	# so keys/values/each work on scalars (experimental)

See also keys, values, and sort.