my $sort_by = 'age';
my $descending = 1;
my $database = \%people;

# return an ordered list of copies of keys of a 
# hash structured as described somewhere
# usage: sort_people( \%sort_me, $by_me, $descending )
# Ascending sort is the default.
# If $by_me is not a key that exists then ...
# Maybe an arref should be returned.
sub sort_people {
    my ( $people, $by, $desc) = @_;
    no warnings qw/ numeric uninitialized/;
    if ( $desc) {
        return sort { your_conditions( $b, $a)} keys %$people;
    } else {
        return sort { your_conditions( $a, $b)} keys %$people;
    }
}