#!/user/bin/perl
use strict;
use warnings;
use Parse::RecDescent;

my $parser = new Parse::RecDescent (q(
    {
        my $operators = {
        '+' => sub { my $i = shift @_; $i += $_ for @_; return $i },
        '-' => sub { my $i = shift @_; $i -= $_ for @_; return $i },
        '*' => sub { my $i = shift @_; $i *= $_ for @_; return $i },
        '/' => sub { my $i = shift @_; $i /= $_ for @_; return $i },
        };
    }
    line: paren /^\Z/
            { $item[1] }
    paren: '(' op arg arg(s) ')'
            { $operators->{$item[2]}->($item[3], @{$item[4]}) }
    op: '+' | '-' | '*' | '/'
    arg: paren | /^[+-]?(?:\d+|\d*\.\d+)/
));

my $task = "(+3 4(*2 7(+1 -1))(/6 2))";
my $calc = $parser->line($task) || die "Syntax error in $task\n";
print $calc, $/;

    op: /[-+*\/](?!\d)/