With IPC::Open3, I can see 2 possible options to see what is actually happening. one is to set the $error filehandle to 0, and redirect it to stdout. The other option to to add both $reader and $error to an IO::Select loop and collect them separately.

Here are 2 samples:

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

my $pid = open3(\*WRITE, \*READ,\*ERROR,"bc");
    #if \*ERROR is false,0,  STDERR is sent to STDOUT 

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

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

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

while(1){

 print "Enter command\n";
 chomp(my $query = <STDIN>);
 
 #send query to bash
 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.
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#!/usr/bin/perl 

# by pg of perlmonks
# This is for lazy people. Sometime
# when you compile your c/c++ program, you may get
# tons of errors, especially when you miss some
# include files or just start a new program. In
# this case the error messages just fly away
# quickly, and you get totally lost. Well, you may
# increase your buffer, you may redirect your
# output but for people who are lazy like me, you
# want a script.

# This script starts your make process thru open3,
# capturing all compiling errors, and dumping them
# to both screen and a file called error_log. If
# the error log is short, you can just view it on
# screen, otherwise vi the error_log.

use IPC::Open3; 
use strict; 
use warnings;

my $pid = open3(\*WRITER, \*READER, \*ERROR,"make"); 
open(ERROR_LOG, ">", "error_log"); 
 while (my $line = <ERROR>) {
      print $line;
      print ERROR_LOG $line; } close(ERROR_LOG);
waitpid($pid, 0);

#######################################################
#or
# "man script"
#or
#  command 2>&1 | tee ./myerror.log
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