Here is some similar code that I wrote a while ago as part of Spreadsheet::WriteExcel::Utility. The code is verbatim although I was tempted to take out some of the parentheses:

    # Convert base26 column string to number
    # All your Base are belong to us.
    my @chars  = split //, $col;
    my $expn   = 0;
    $col       = 0;

    while (@chars) {
        my $char = pop(@chars); # LS char first
        $col += (ord($char) -ord('A') +1) * (26**$expn);
        $expn++;
    }
[download]

As an aside. Any particular reason for using POSIX::pow()?

And as another aside the POSIX strtol() function will convert base 26 (or bases from 2 to 36) to base 10, although not for this base 26 format.

--
John.