It might help to realize that the column name is essentially a base-26 number, using A as '0'. Then a div+mod loop while anything is left would only require building a 26 character alphabet. (for all I know, this could be how the XLS utils mentioned above handle it).

There are a couple of fencepost errors here. Fix coming shortly Ok, this is not quite a base-26 number. The least significant digit is treated slightly different than all other digits.

sub mlx_xls2col {
    my $col      = shift;
    my $alphabet = shift || [ 'A' .. 'Z' ];

    my $alphabet_size = scalar( @$alphabet );

    my $result    = '';
    my $remainder = $col;
    while ( $remainder ) {
        my $letter = $remainder % $alphabet_size;
        my $adj = length( $result ) ? -1 : 0;    # Not quite a base-26
+ number
        $result = $alphabet->[$letter + $adj] . $result;
        $remainder = int( $remainder / $alphabet_size );
    }
    $result ||= $alphabet->[0];

    $result;
}

print mlx_xls2col( 0 ), "\n";
print mlx_xls2col( 1 ), "\n";
print mlx_xls2col( 2 ), "\n";
print mlx_xls2col( 24 ), "\n";
print mlx_xls2col( 25 ), "\n";
print mlx_xls2col( 26 ), "\n";
print mlx_xls2col( 27 ), "\n";
print mlx_xls2col( 51 ), "\n";
print mlx_xls2col( 52 ), "\n";
print mlx_xls2col( 53 ), "\n";

__DATA__
A
B
C
Y
Z
AA
AB
AZ
BA
BB
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