Programming Perl, 4th Edition

keys

keys HASH
keys ARRAY
keys EXPR

This function returns a list consisting of all keys in the indicated HASH. The keys are returned in an apparently random order, but it is the same order produced by either the values or each function (assuming the hash has not been modified between calls). As a side effect, it resets HASH’s iterator. Here is a (rather cork-brained) way to print your environment:

@keys   = keys   %ENV;    # keys are in the same order as
@values = values %ENV;    # values, as this demonstrates
while (@keys) {
    say pop(@keys), "=", pop(@values);
}

You’re more likely to want to see the environment sorted by keys:

for my $key (sort keys %ENV) {
    say $key, "=", $ENV{$key};
}

You can sort the values of a hash directly, but that’s somewhat useless in the absence of any way to map values back to keys. To sort a hash by value, you generally need to sort the keys by providing a comparison function that accesses the values based on the keys. Here’s a descending numeric sort of a hash by its values:

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

Using keys on a hash bound to a largish DBM file will produce a largish list, causing you to have a largish process. You might prefer to use the each function here, which will iterate over the hash entries one by one without slurping them all into a single gargantuan list.

In scalar context, keys returns ...