my @names = qw/andy betty craig daniel edith/;
my %reverse_lookup;
@reverse_lookup{ @names } = (0..4);

foreach $year (2004 .. 2007) {
    foreach (sort keys %people) {
        printf("%04d: %s buys for %s\n", 
            $year, 
            $_, 
            $names[ ($year+$reverse_lookup{$_}) % 4 + 1]
        );
    }
}