It sounds like you just want to be able to add some sort of constant to your a string. You can't really do that; however, here is a function that will give you want you want:

sub magic
{
    my ($v, $i) = @_;
    $v=substr($v,0,length($v)-1).chr(ord(substr($v,-1))+$i);
}

print magic('a',  1), ", ";
print magic('c', -1), ", ";
print magic('meep', 2);

# will print "c, a, meer"
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