$Data{gen} = {
   'beholder' => {b => 1, pg => 21},
   'death kiss' => {pg => 21},
   ...
};

##</code><code>##

sub random
{
    my ( $key ) = @_;
    my @choices = keys %{$Data{$key}};
    my $chosen = $choices[rand @choices];
    return $Data{$key}{$chosen};  # or maybe just return $chosen ?
}

##</code><code>##

my $output = my $randomkey = random( "gen" );

##</code><code>##

$output .= " beholder" unless ( exists( $Data{gen}{$randomkey}{b} ));

##</code><code>##

my $addon = '';
if ( exists( $Data{gen}{$randomkey}{bk} ) or
     exists( $Data{gen}{$randomkey}{pg} ))
{
    $addon = $Data{gen}{$randomkey}{bk} || 'Monstrous Manual';
    $addon .= ", page $Data{gen}{$randomkey}{pg}"
        if ( exists( $Data{gen}{$randomkey}{pg} ));
}
$output .= " ($addon)" if ( $addon );