YES YES YES!!!

No matter how sure I was about the data-structure being properly created, I decided to out code like inside my loop:

if ( exists( $data{$array[$i]}{$array[$j]} ) ) {
        next();
        }
else {
        print OUT join("\t", $i, $j), "\n";
        }
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and... wouldn't ya know it, there *were* elements being autovivified. I've fixed that problem, and now the code appears to be running correctly (appears = its been running for a day without crashing yet -- it still has another couple of days to run, though).

Many thanks -- this has led me to one key problem in the code. Any monks who haven't done so yet, please ++ the parent.