The behaviour is the same as if you increment a number, only the "numbers" are not 0-9, but a-z. The simplest way to see that is to start "counting" from "a".

$s = "a";
for (1..702) { print $s++, "\n"; }
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prints

a
b
c
...
z
aa
ab
ac
...
az
ba
bb
bc
...
zz
[download]