# THE COMMONEST REASON FOR WANTING LEFT RECURSION

use strict;
use Parse::RecDescent; $::RD_HINT = 1;


my $parse = Parse::RecDescent->new(<<'EndGrammar');

    main: expr /\Z/ { $item[1] }
        | <error>

    expr: left_assoc[qw{term add_op term}]
        | term

    add_op: '+'    { sub { $_[0] + $_[1] } }
          | '-'    { sub { $_[0] - $_[1] } }

    term: left_assoc[qw{factor mult_op factor}]
        | factor

    mult_op: '*'    { sub { $_[0] * $_[1] } }
           | '/'    { sub { $_[0] / $_[1] } }

    factor: number
          | '(' expr ')' { $item[2] }

    number: /[-+]?\d+(\.\d+)?/


# THE BLACK MAGIC THAT MAKES IT WORK...

    left_assoc: left_assoc_left[@arg[0,1]](s) <matchrule:$arg[2]>
            { my @terms = $item[1]
                ? ((map { @$_ } @{$item[1]}), $item[2])
                : $item[1];
              splice @terms, 0, 3, $terms[1]->(@terms[0,2])
                while @terms>1;
              $terms[0];
            }

    left_assoc_left: <matchrule:$arg[0]> <matchrule:$arg[1]>
            { [ @item[1..2] ] }


EndGrammar

while (<DATA>) {
  print $parse->main($_), "\n";
}

__DATA__
+1-1+1-1+1-1+1-1+1
7*7-6*8
121/(121/11)/121*11
1/(10-1/(1/(10-1)))