my $str = 'aaaa';
print $str++, $/ while $str le 'dddd';

##</code><code>##

my $str = 'aaaa';
while ($str le 'dddd') {
    print $str++, $/;
    1 while $str =~ s/(.)e/chr(1 + ord $1) . 'a'/e;
}