=over

=item keys HASH

Returns a list consisting of all the keys of the named hash.
(In scalar context, returns the number of keys.)

The keys 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 the same order as either the C<values> or C<each>
function produces (given that the hash has not been modified).  Since
Perl 5.8.1 the ordering is different even between different runs of
Perl for security reasons (see L<perlsec/"Algorithmic Complexity
Attacks">).

As a side effect, calling keys() resets the HASH's internal iterator,
see L</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 L</values>.

To sort a hash by value, you'll need to use a C<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;
    }

As an lvalue C<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 C<%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 C<%hash = ()>, use C<undef
%hash> if you want to free the storage while C<%hash> is still in scope.
You can't shrink the number of buckets allocated for the hash using
C<keys> in this way (but you needn't worry about doing this by accident,
as trying has no effect).

See also C<each>, C<values> and C<sort>.

=back