The auto-increment operator has a little extra builtin
magic to it.  If you increment a variable that is numeric,
or that has ever been used in a numeric context, you get a
normal increment.  If, however, the variable has been used
in only string contexts since it was set, and has a value
that is not the empty string and matches the pattern
`/^[a-zA-Z]*[0-9]*$/', the increment is done as a string,
preserving each character within its range, with carry:

           print ++($foo = '99');      # prints '100'
           print ++($foo = 'a0');      # prints 'a1'
           print ++($foo = 'Az');      # prints 'Ba'
           print ++($foo = 'zz');      # prints 'aaa'