Please consider using a regular expression to perform the validation as it is much simpler to code and faster to execute:

use feature ":5.14";
use warnings FATAL => qw(all);
use strict;
use Data::Dump qw(dump);

say (/\Aa*\Z/ ? "$_ is all a's" : "$_ is not all a's") for qw(aaaa aaa
+ab aaac cccccbbb cccca);
[download]

Produces:

aaaa is all a's
aaaab is not all a's
aaac is not all a's
cccccbbb is not all a's
cccca is not all a's