The following still takes 90s for size 10_000, and 800s for size 20_000 on my machine (with some random tuning of the LOAD_AT_ONCE constant). 640_000 would still take several years. Nevertheless, I haven't been able to find a faster solution.

<Reveal this spoiler or all in this thread>

I used the following code to generate the input matrix: 

my $SIZE = 1000;
sub create_matrix {
    my ($filename) = @_;
    open my $OUT, '>', $filename or die $!;

    for my $i (1 .. $SIZE) {
        for my $j ( 1 .. $SIZE ) {
            print {$OUT} $i <= $j ? $i * $j : 'NA';
            print {$OUT} ' ' unless $SIZE == $j;
        }
        print {$OUT} "\n";
    }
}

[download]

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,

[download]

In reply to Re^2: upper or lower triangular matrix to full by choroba
in thread upper or lower triangular matrix to full by savita

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