$s = "a";
for (1..702) { print $s++, "\n"; }
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a
b
c
...
z
aa
ab
ac
...
az
ba
bb
bc
...
zz
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as for the "increment the whole string" comment, you are doing. If you increment the number 4356 to 4357, you only change one 'character', but you are operating on the whole 'string'. This is exactly the same for your example, where only the last character has rolled over, which increments the last but one. If you want to increment each character in the string, you may want to do something like this.

$sample='prad';
print join('', map { ++$_} split(//, $sample) );
print"$sample";
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The behavior is not unusual. If you incremented 1111 you would get 1112, and if you incremented it ten times you would get 1121. Eleven times would give you 1122. A hundred times would give you 1211... you get the idea.

So, given that alpha characters can also be incremented, it should come as no surprise that they're incremented right to left also; consider the right-most digit to be the least significant digit, whether it's a number or an alpha character. Thus, 'aaaa' would become 'aaab'. Increment it enough times, it it becomes 'aaba', 'aabb', 'aabc' ... 'aaca', 'aacb', 'aacc', and so on, until you finally arrive at 'zzzz'.

If you really just wanted to increment each character by one, you might consider constructing a regexp like this:

$string =~ s/(.)/$char = $1;++$char/eg;
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