The error message is right. Instead of this, which makes a string:

push(@$outputRef, "$symbol,$side,$account,$shares");
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you need this, which makes an arrayref:

push(@$outputRef, [ $symbol,$side,$account,$shares ]);
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or to make everything more readable further down:

push(@$outputRef, {
    symbol => $symbol,
    side => $side,
    account => $account,
    shares => $shares,
});
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But if that's the code you can't change, you'll have to split the strings first:

my @sorted = sort{
    $a->[1] cmp $b->[1] || $a->[0] cmp $b->[0] || $a->[2] cmp $b->[2]
} map { [split(',', $_)] } @$outputref;
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update: removed mistaken notion of using a simple string sort.