3.2.79 keys

keys 

HASH

This function returns a list consisting of all the keys of the named hash. The keys are returned in an apparently random order, but it is the same order as either the values or each function produces (assuming that the hash has not been modified between calls). 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";
}

To sort a hash by value, you'll need to provide a comparison 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;
}

Note that 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 in this case, which will iterate over the hash entries one-by-one without slurping them all into a single gargantuan list.

In scalar context, keys returns the number of elements of the hash (and resets the each iterator). However, to get this information for tied hashes, including DBM files, Perl must still walk the entire hash, so it's not very efficient in that case.