sub decrement_word {
    my ($pword) = @_;
    my @vals = map { ord $_ } (split //, $$pword);
    my $idx = $#vals;
    while (1) {
        if ($vals[$idx] != ord('a') and $vals[$idx] != ord('A')) {
            --$vals[$idx];
            last;
        }
        if (0 == $idx) {
            shift @vals;
            last;
        }
        $vals[$idx] += 25;
        --$idx;
    }
    my $word = join '', map { chr } @vals;
    $$pword = $word;
}