perldoc -f substr

Alternatively you can use split but if you want changes to the array to be reflected back in the string, you'll need to reform the string:

@string = split //, $string;
# manipulate @string all you want
$string = join '', @string;
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update: BTW, I forgot to mention, if you're trying to validate your data as something generally well known, there are modules on CPAN that can probably do it for you.