Perl5 リファレンス

       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 `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) {
                       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 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).

               See also `each', `values' and `sort'.