use strict;
use warnings;

use constant OFFS => 1 - ord 'a';
use constant BASE => ord("z") - ord("a") + 1;

sub alphabetic_to_base10 {
    local $_ = shift;
    my $num = 0;
    $num = ($num * BASE) + (OFFS + ord $_) for /./g;
    return $num;
}

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
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Makeshifts last the longest.