Not as short but a bit clearer (to me, anyway) than the others I've seen:

sub b26_to_b10
{
   my @digits = reverse split //, shift;
   my $i = 1;
   my $result = 0;
   for ( @digits ) {
       $result += ( ord(lc($_)) - ord('a') + 1 ) * $i;
       $i *= 26;
   }
   return $result;
}
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Christopher E. Stith