#!/usr/bin/perl
use warnings;
use strict;
use IPC::Open3;

#interface to "bc" calculator
#my $pid = open3(\*WRITE, \*READ, \*ERROR,"bc");
my $pid = open3(\*WRITE, \*READ,0,"bc");
            #if \*ERROR is false, STDERR is sent to STDOUT 
while(1){
print "Enter expression for bc, i.e. 2 + 2\n";
chomp(my $query = <STDIN>);

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

#give bc time to output
select(undef,undef,undef,.5);

#get the answer from bc
chomp(my $answer = <READ>);
print "$query = $answer\n";

}

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

#!/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.
[download]