When called in list context, returns a 2-element list consisting of the key and value for the next element of a hash, so that you can iterate over it. When called in scalar context, returns the key for only the "next" element in the hash. (Note: Keys may be "0" or "", which are logically false; you may wish to avoid constructs like while ($k = each %foo) {} for this reason.)
Entries are returned in an apparently random order. The actual random order is subject to change in future versions of perl, but it is guaranteed to be in the same order as either the keys() or values() function would produce on the same (unmodified) hash.
When the hash is entirely read, a null array is returned in list context (which when assigned produces a FALSE (0) value), and undef in scalar context. The next call to each() after that will start iterating again. There is a single iterator for each hash, shared by all each(), keys(), and values() function calls in the program; it can be reset by reading all the elements from the hash, or by evaluating keys HASH or values HASH. If you add or delete elements of a hash while you're iterating over it, you may get entries skipped or duplicated, so don't.
The following prints out your environment like the printenv(1) program, only in a different order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}See also keys(), values() and sort().