expr     : bin_op_2 { $item[1] }

   bin_op_2 : bin_op_2 /[+-]/ bin_op_1  { [ @item[2, 1, 3] ] }
            | bin_op_1                  { $item[1] }

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

##</code><code>##

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

   bin_op_1  : term bin_op_1_(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] ] }

##</code><code>##

      sub treeify {
         my $t = shift;
         $t = [ shift, $t, shift ]
            while @_;
         return $t;
      }

##</code><code>##

   # Lowest precedence.
   bin_op_2 : <leftop: bin_op_1 SUM bin_op_1 >
              { treeify(@{$item[1]}); }

   bin_op_1 : <leftop: term PROD term >
              { treeify(@{$item[1]}); }
   # Highest precedence.

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

##</code><code>##

use strict;
use warnings;

use Data::Dumper      ();
use Parse::RecDescent ();


my $grammar = <<'__EOI__';

   {
      use strict;
      use warnings;

      sub treeify {
         my $t = shift;
         $t = [ shift, $t, shift ]
            while @_;
         return $t;
      }
   }

   parse   : expr EOF { $item[1] }

   expr    : assign { $item[1] }


   # Lowest precedence.
   assign  : IDENT '=' assign { [ $item[2], $item[1], $item[3] ] }
           | log_or           { $item[1] }
   log_or  : <leftop: log_and LOG_OR  log_and > {treeify(@{$item[1]})}
   log_and : <leftop: sum     LOG_AND sum     > {treeify(@{$item[1]})}
   sum     : <leftop: prod    SUM     prod    > {treeify(@{$item[1]})}
   prod    : <leftop: term    PROD    term    > {treeify(@{$item[1]})}
   # Highest precedence.

   term    : '(' expr ')'   { $item[2] }
           | NUMBER         { [ $item[0], $item[1] ] }


   # Tokens

   NUMBER   : /\d+/
   IDENT    : /\w+/

   LOG_OR   : '||' 
   LOG_AND  : '&&'
   SUM      : '+' | '-'
   PROD     : '*' | '/' | '\\' | '%'

   EOF      : /^\Z/

__EOI__


# $::RD_HINT  = 1;
# $::RD_TRACE = 1;

my $parser = Parse::RecDescent->new($grammar)

or die("Bad grammar\n");

foreach (
  '1*2*3',
  '1%2*3',
  '1+2*3',
  'a=b=c=1',
) {
   print(Data::Dumper->Dump([ $parser->parse($_) ],
                            [ $_                 ]));
   print("\n");
}

##</code><code>##

      sub treeify_r {
         my $t = pop;
         $t = [ pop, pop, $t ]
            while @_;
         return $t;
      }

   assign   : <rightop: IDENT ASSIGN assign >
              { treeify_r(@{$item[1]}); }

   ASSIGN   : '='