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 ($k = each %foo) {} for this reason.)
Entries are returned in an apparently random order. When the hash is entirely read, a null array is returned in list context (which when assigned produces a FALSE (0) value), and undef in scalar context. The next call to each() after that will start iterating again. There is a single iterator for each hash, shared by all each(), keys(), and values() function calls in the program; it can be reset by reading all the elements from the hash, or by evaluating keys HASH or values HASH. If you add or delete elements of a hash while you're iterating over it, you may get entries skipped or duplicated, so don't.
The following prints out your environment like the printenv(1) program, only in a different order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}See also keys() and values().