#!/usr/bin/env perl
    use utf8;
    use Unicode::Collate;

    my $uc = Unicode::Collate->new(
        normalization => undef, level => 1
    );
    printf "pos %d len %d\n",
        $uc->index('Lettre à ma sœur.docx', $_)
        for 'sœur', 'soeur', 'lettre   a ma-soeur _DOCX_';
    __END__
    pos 12 len 4
    pos 12 len 4
    pos 0 len 21

You have been wrong three times in a row now, just because you have an insufficient understanding of the standard.

Time to do what I said in the beginning and read it, or do you want to continue arguing against straw-men and pretending to be knowledgeable enough about the subject matter to talk about it authoritatively?