for my $n (25..28) {
	printf"%s\n",pack"C*",map{$_%26+ord'a'}0..$n-1; # ok
	printf"%s\n",pack"C*",map{ord'a'+$_%26}0..$n-1; # not ok
}

__END__

abcdefghijklmnopqrstuvwxy
0123456789111111111122222
abcdefghijklmnopqrstuvwxyz
01234567891111111111222222
abcdefghijklmnopqrstuvwxyza
012345678911111111112222220
abcdefghijklmnopqrstuvwxyzab
0123456789111111111122222201