use List::Util qw(shuffle);

sub random {
  my ($user_input, $list) = @_;
  
  $list->{'all'}  = [ map { @$_ } values %{$list} ];
  $list->{'keys'} = [ grep {$_ !~ /(?:all|keys)/} keys %{$list} ];
  
  if ($user_input && $user_input =~ /(?:help|options)/) {
    return 'Your options are: '.join(', ',sort @{$$list{'keys'}}).', or all.';
  }
  elsif ($user_input && $user_input =~ /dump/) {
    use Data::Dumper;
    return Dumper($list);
  }
  else {
    my $input = $user_input ? $user_input : 'all';
    my @random_list = shuffle(@{$$list{$input}});
    return $random_list[rand @random_list];
  }
}

##</code><code>##

my %waters = (
  running  => [qw(spring streamlet rivulet run brook creek stream river)],
  standing => [qw(drop puddle pool pond lake sea ocean)],
  precipitation => [qw(rain snow sleet hail)]
);

sub random_water {
  my ($user_water) = @_;
  random($user_water, \%waters);
}

##</code><code>##

print random_water('dump');

$VAR1 = {
          'keys' => [
                      'standing',
                      'precipitation',
                      'running'
                    ],
          'standing' => [
                          'drop',
                          'puddle',
                          'pool',
                          'pond',
                          'lake',
                          'sea',
                          'ocean'
                        ],
          'precipitation' => [
                               'rain',
                               'snow',
                               'sleet',
                               'hail'
                             ],
          'running' => [
                         'spring',
                         'streamlet',
                         'rivulet',
                         'run',
                         'brook',
                         'creek',
                         'stream',
                         'river'
                       ],
          'all' => [
                     'drop',
                     'puddle',
                     'pool',
                     'pond',
                     'lake',
                     'sea',
                     'ocean',
                     'rain',
                     'snow',
                     'sleet',
                     'hail',
                     'spring',
                     'streamlet',
                     'rivulet',
                     'run',
                     'brook',
                     'creek',
                     'stream',
                     'river'
                   ]
        };

##</code><code>##

print random_water('dump');
print random_water('dump');

$VAR1 = {
          'keys' => [
                      'standing',
                      'precipitation',
                      'running'
                    ],
          'standing' => [
                          'drop',
                          'puddle',
                          'pool',
                          'pond',
                          'lake',
                          'sea',
                          'ocean'
                        ],
          'precipitation' => [
                               'rain',
                               'snow',
                               'sleet',
                               'hail'
                             ],
          'running' => [
                         'spring',
                         'streamlet',
                         'rivulet',
                         'run',
                         'brook',
                         'creek',
                         'stream',
                         'river'
                       ],
          'all' => [
                     'standing',
                     'precipitation',
                     'running',
                     'drop',
                     'puddle',
                     'pool',
                     'pond',
                     'lake',
                     'sea',
                     'ocean',
                     'rain',
                     'snow',
                     'sleet',
                     'hail',
                     'spring',
                     'streamlet',
                     'rivulet',
                     'run',
                     'brook',
                     'creek',
                     'stream',
                     'river',
                     'drop',
                     'puddle',
                     'pool',
                     'pond',
                     'lake',
                     'sea',
                     'ocean',
                     'rain',
                     'snow',
                     'sleet',
                     'hail',
                     'spring',
                     'streamlet',
                     'rivulet',
                     'run',
                     'brook',
                     'creek',
                     'stream',
                     'river'
                   ]
        };