Simple and elegant given that Excel has a max column of "XFD". I'd modify this to cache the column names and throw a fit if we go too far (eventually, the column cap will be increased or removed):

sub ltr_cached {
    state $ltr_cache = ["A".."ZZZ"];
    return ($$ltr_cache[$_[0]] or die "column too big: $_[0]")
}
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