keys

See the current Perl documentation for keys.

Here is our local, out-dated (pre-5.6) version:

keys - retrieve list of indices from a hash

keys HASH

Returns a list consisting of all the keys of the named hash. (In a scalar context, returns the number of keys.) The keys are returned in an apparently random order, but it is the same order as either the values() or each() function produces (given that the hash has not been modified). As a side effect, it resets HASH's iterator.

Here is yet another way to print your environment:

    @keys = keys %ENV;
    @values = values %ENV;
    while ($#keys >= 0) {
        print pop(@keys), '=', pop(@values), "\n";
    }

or how about sorted by key:

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

To sort an array 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;
    }

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).