TMTOWTDI

$ perl -E '
    my @array = (1,1,1,2,2,2,2,2,3,3,3,4,4,4,4,4,4,5,5,6);
    {
        local $" = "";
        say rindex "@array", "4";
    }
    say "@array[16, 17]";
'
16
4 5
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In your example data, you show only elements with single digits. The solution I presented assumes this is representative data.