use strict;
use warnings;
use List::Util qw(sum);
use constant OFFS => 1 - ord 'a';
use constant BASE => ord("z") - ord("a") + 1;

sub alphabetic_to_base10 {
    local $_ = reverse shift;
    my $fac = 1;
    sum map +( (OFFS+ord $_)*$fac, $fac*=BASE )[0], /./g;
}

printf "$_: %d\n", alphabetic_to_base10($_) for qw[a z aa az ba zz aaa zba];
__END__
a: 1
z: 26
aa: 27
az: 52
ba: 53
zz: 702
aaa: 703
zba: 17629