Using the builtin "leftop" directive speeds things up quite a bit, e.g.:

lv1expr : <leftop:lv2expr addsub lv1expr>  {
        my @ops = @{$item[1]};
        my $res = shift @ops;
        while (@ops) {
                my ($op, $num) = (shift@ops, shift@ops);
                if ($op eq '+') { $res+= $num }
                else { $res -= $num; }
        }
        $return = $res;
    }
# ...
addsub: /[-+]/
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/s