use Parse::RecDescent;

sub evalop
{
    my (@list) = @{[@{$_[0]}]};
    my $val = shift(@list)->();
    while (@list)
    {
        my ($op, $arg2) = splice @list, 0, 2;
        $op->($val,$arg2->());
    }
    return $val;
}

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

    main: expr /\s*\Z/ { $item{expr}->() }
        | <error: expected expression in $item{__RULE__}>

    expr: /for(each)?/ lvar range expr
            { my ($vname,$expr) = @item{"lvar","expr"};
              my ($from, $to) = @{$item{range}};
              sub { my $val;
                no strict "refs";
                for $$vname ($from->()..$to->())
                    { $val = $expr->() }
                return $val;
                  }
            }
        | lvar '=' addition
            { my ($vname, $expr) = @item{"lvar","addition"};
              sub { no strict 'refs'; $$vname = $expr->() }
            }
        | addition

    range: "(" expr ".." expr ")"
            { [ @item[2,4] ] }

    addition: <leftop:multiplication add_op multiplication>
            { my $add = $item[1]; sub { ::evalop $add } }

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

    multiplication: <leftop:factor mult_op factor>
            { my $mult = $item[1]; sub { ::evalop $mult } }

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

    factor: number
          | rvar
          | '(' expr ')' { $item{expr} }

    number: /[-+]?\d+(\.\d+)?/    { sub { $item[1] } }

    lvar:    /\$([a-z]\w*)/        { $1 }

    rvar:    lvar            { sub { no strict 'refs'; ${$item{lvar}} } }

EndGrammar

print "> ";
while (<>) {    # FOR DEMO CHANGE TO: while (<DATA>)
  print eval {$parse->main($_)}||"", "\n\n> ";
}

__DATA__
$x = 2
$y = 3
+1-1+1-1+1-1+1-1+1
7*7-6*8
121/(121/11)/121*11
1/(10-1/(1/(10-1)))
$x * $y
foreach $i (1..$y) $x = $x * 2 + $i
$x