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keys HASH
keys ARRAY
keys EXPR

Called in list context, returns a list consisting of all the keys of the named hash, or in Perl 5.12 or later only, the indices of an array. Perl releases prior to 5.12 will produce a syntax error if you try to use an array argument. In scalar context, returns the number of keys or indices.

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. Tied hashes may behave differently to Perl's hashes with respect to changes in order on insertion and deletion of items.

As a side effect, calling keys() resets the internal iterator of the HASH or ARRAY (see "each"). In particular, calling keys() in void context resets the iterator with no other overhead.

Here is yet another way to print your environment:

@keys = keys %ENV;
@values = values %ENV;
while (@keys) {
    print pop(@keys), '=', pop(@values), "\n";

or how about sorted by key:

foreach $key (sort(keys %ENV)) {
    print $key, '=', $ENV{$key}, "\n";

The returned values are copies of the original keys in the hash, so modifying them will not affect the original hash. Compare "values".

To sort a hash by value, you'll need to use a sort function. Here's a descending numeric sort of a hash by its values:

foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
    printf "%4d %s\n", $hash{$key}, $key;

Used as an lvalue, keys allows you to increase the number of hash buckets allocated for the given hash. This can gain you a measure of efficiency if you know the hash is going to get big. (This is similar to pre-extending an array by assigning a larger number to $#array.) If you say

keys %hash = 200;

then %hash will have at least 200 buckets allocated for it--256 of them, in fact, since it rounds up to the next power of two. These buckets will be retained even if you do %hash = (), use undef %hash if you want to free the storage while %hash is still in scope. You can't shrink the number of buckets allocated for the hash using keys in this way (but you needn't worry about doing this by accident, as trying has no effect). keys @array in an lvalue context is a syntax error.

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

for (keys $hashref) { ... }
for (keys $obj->get_arrayref) { ... }

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 each, values, and sort.