May I offer a slightly more general alternative to your sub colIdToString:

sub colIdToString {
    my $col = shift;
    my $column = "";
    while( $col ) {
        my $remainder = ($col-1) % 26 + 1;
        $column = chr( $remainder + 64 ).$column;
        $col -= $remainder;
        $col /= 26;
    }
    return $column;
}
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