The basic base-26 conversion doesn't work right immediately (you get ...Y,AZ,AA,AB...), but a well placed decrement fixes it
sub ConvertToAlpha {
    my $self = shift;
    my $inNumber = shift;
    my @output = ();
    while ($inNumber > 0) {
        unshift(@output, ($inNumber % 26));
        my $shouldDecrement = ($inNumber %26 == 0);
        use integer;
        $inNumber = $inNumber / 26;
        if ($shouldDecrement) {
            $inNumber--;
        }
    }
    # @output-1 since we won't ever need to change the last digit
    my %toAlpha = ( 1=>'A',        2=>'B',        3=>'C',
            4=>'D',        5=>'E',        6=>'F',
            7=>'G',        8=>'H',        9=>'I',
            10=>'J',    11=>'K',    12=>'L',
            13=>'M',    14=>'N',    15=>'O',
            16=>'P',    17=>'Q',    18=>'R',
            19=>'S',    20=>'T',    21=>'U',
            22=>'V',    23=>'W',    24=>'X',
            25=>'Y',    0=>'Z');
    foreach (@output) {
        $_=$toAlpha{$_};
    }
    join("",@output);

}

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In reply to Converting numbers to Excel Column indices by Willworker

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