#!/usr/bin/perl

# sgifford of perlmonks 
# The reason for your problem is because you're not using  
# IO::Select quite right. You're passing a timeout of 0 to  
# can_read, which asks it to return immediately if there is  
# no data ready yet. What I think you want to do is create  
# one IO::Select object with both handles, then ask it to  
# wait until one or both have something ready to read.  

# Something like this: 
# It's only drawback is it only outputs 1 line of bc output 
# so it errs on something like 234^12345 (which outputs a big number) 

use warnings;
use strict;
use IPC::Open3;
use IO::Select;

#interface to "bc" calculator 
my $pid = open3(\*WRITE, \*READ,\*ERROR,"bc");

my $sel = new IO::Select();

$sel->add(\*READ);
$sel->add(\*ERROR);

my($error,$answer)=('','');

while(1){

 print "Enter expression for bc, i.e. 2 + 2\n";
 chomp(my $query = <STDIN>);

 #send query to bc 
 print WRITE "$query\n";

 foreach my $h ($sel->can_read)
 {
   my $buf = '';
   if ($h eq \*ERROR)
   {
     sysread(ERROR,$buf,4096);
     if($buf){print "ERROR-> $buf\n"}
   }
   else
   {
     sysread(READ,$buf,4096);
     if($buf){print "$query = $buf\n"}
   }
 }
}

waitpid($pid, 1);
# It is important to waitpid on your child process,   
# otherwise zombies could be created.