#!/usr/bin/perl -w
use strict;

sub convert26 {
    my $v = 0;
    $v = ($v * 26 + $_ - ord('a') + 1) for unpack("C*",shift);
    $v;
}

while (<DATA>) {
    chomp;
    print "$_: ",convert26($_),$/;
}

__DATA__
a
z
aa
az
ba
zz
aaa
[download]

Output:

a: 1
z: 26
aa: 27
az: 52
ba: 53
zz: 702
aaa: 703
[download]