perl char

why the 1st one output is 'aa' ?
the 1st one you add 1 to charater 'z'
why equal to 'aa' then?

why the 2nd one output is 'b'?
how did it gets 'b'?

=============
our $v = 'z';
$v++;
print $v;

=============

our $x = 'a';
$x++;
print $x;

=============

thanks
.