$val = 'BB';
incr($val);  ==> 'BC'

$val = 'BZ';
incr($val);  ==> 'CB';

$val = 'ZZ';
incr($val)  ==> 'BBB';

##</code><code>##

use 5.010;
use strict;
use warnings;

my $x = 'ZX';
for my $i ( 1 .. 100 ) {
    $x = incra($x);
    say $x;
}

sub incra {
    my $val = shift or die "Where's my value?";
    $val = reverse $val;    # easier to think left to right
    my $digits  = 'BCDFGHJKLMNPQRSTVWXZ';
    my $ndigits = length $digits;

    my $carry_in = 1;
    my $result   = '';
    for my $digit ( split //, $val ) {
        my $i = index $digits, $digit;
        if ($carry_in) {
            if ( $i == $ndigits - 1 ) {
                $digit = substr $digits, 0, 1;
            }
            else {
                $digit = substr $digits, $i + 1, 1;
                $carry_in = 0;
            }
        }
        $result .= $digit;

    }
    $result .= substr $digits, 0, 1 if $carry_in;
    return reverse $result;
}