How can I access/change the first N letters of a string?

There are many ways. If you just want to grab a copy, use substr: 

    $first_byte = substr($a, 0, 1);

If you want to modify part of a string, the simplest way is often to use substr as an lvalue: 

    substr($a, 0, 3) = "Tom";

Although those with a regexp kind of thought process will likely prefer 

    $a =~ s/^.../Tom/;