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   expr     : bin_op_2 { $item[1] }

...

   bin_op_1 : bin_op_1 /[*\\\/%]/ term  { [ @item[2, 1, 3] ] }
            | term                      { $item[1] }

   bin_op_2  : bin_op_1 bin_op_2_(s?)
               { treeify($item[1], map { @$_ } @{$item[2]}); }
...

   bin_op_2_ : /[+-]/     bin_op_1 { [ $item[1], $item[2] ] }
   bin_op_1_ : /[*\\\/%]/ term     { [ $item[1], $item[2] ] }

      sub treeify {
         my $t = shift;
...
            while @_;
         return $t;
      }

   # Lowest precedence.
   bin_op_2 : <leftop: bin_op_1 SUM bin_op_1 >
...

   SUM      : '+' | '-'
   PROD     : '*' | '/' | '\\' | '%'

use strict;
use warnings;
...
                            [ $_                 ]));
   print("\n");
}

      sub treeify_r {
         my $t = pop;
...
              { treeify_r(@{$item[1]}); }

   ASSIGN   : '='