my $token_count = 0;
while (@tokens) {
    my $token = shift @tokens;
    # if its an opening paran then ...
    if ($token eq '(') {
        push(@operator_stack, $token);
    }		
    # if it's an operator then ...
    elsif (is_operator($token)) {
        push(@operator_stack, $token);
    }
    # if its a closing paran then ...
    elsif ($token eq ')') {
        # empty the operator stack until
        # we find the opening paren
        while (@operator_stack && $operator_stack[-1] ne '(') {
            # and push everything onto the
            # operator stack
            push @program_stack, (pop @operator_stack); 
        }
        # then pop the opening paren off
        pop @operator_stack;
    }
    # if its not an operator, or a paran ...
    else {
        push @program_stack, $token;
        $token_count++;
        # if we have 2 or more tokens on the
        # program stack (since out last operator)
        # we need to get an operator ....
        if ($token_count >= 2){
            # unless the top of our operator stack
            # is a opening paran, in which case
            # we have a sub-expression which needs
            # to be handled first
            next if $operator_stack[-1] eq '(';
            # otherwise grab the operator, and ...
            push @program_stack, (pop @operator_stack);
            # reset the token count
            $token_count = 1;
        }
    }
}
# and lastly, empty whats left of your 
# operator stack onto your program stack
push @program_stack, (pop @operator_stack) while (@operator_stack);