#!/usr/bin/perl

use warnings;

use IPC::Open3;
use IO::Select;
use POSIX ":sys_wait_h";
use Symbol;

sub run
{
  my ($WRITE, $READ, $ERROR);
  $ERROR=gensym();
  my $command="not_a_command";
  my $error="";
  my $output="";
  eval { $pid=open3($WRITE, $READ, $ERROR, "$command"); };
  if (!$pid) {
      # if we get here, the fork succeeded, but the exec (likely) failed...
      my $err = $@ ? $@ : "unknown error";
      # exit child in any case
      die "Error: Could not execute $command: $err";
  } else {
    close($WRITE);
    my $selector=IO::Select->new();
    $selector->add($READ, $ERROR);
    while(@ready=$selector->can_read)
    {
      foreach (@ready)
      {
        if(fileno($_)==fileno($READ))
        {
          $bytes=sysread($READ, $text, 1024);
          if($bytes==0) { $selector->remove($_); }
          else { $output.=$text; }
        }
        elsif(fileno($_)==fileno($ERROR))
        {
          $bytes=sysread($ERROR, $text, 1024);
          if($bytes==0) { $selector->remove($_); }
          else { $error.=$text; }
        }
      }
    }
    waitpid($pid, 0);
  }
  return($output, $error);
}

my ($output, $error);
($output, $error)=run();
print "$output, $error\n";


##</code><code>##

$ ./test_eval.pl
, Error: Could not execute not_a_command: open3: exec of not_a_command failed at ./test_eval.pl line 22

##</code><code>##

  ...
  if (!$pid) {
      # if we get here, the fork succeeded, but the exec (likely) failed...
      my $err = $@ ? $@ : "unknown error";
      # return error, and exit child
      print STDERR "Error: Could not execute $command: $err";
      POSIX::_exit($!);
  }
  ...