I think that you were missing a few things, primarily a reaper subroutine---without that, it'll always return -1. Here's how I would do it:

!/usr/bin/perl

use strict;
use warnings;
use IO::CaptureOutput qw(capture capture_exec);
use Proc::Killall;
use POSIX qw(:signal_h :errno_h :sys_wait_h);

&runCommand();

my $pid;
unless ( $pid = fork ) {
    &runCommand();
    exit;
}

$SIG{'CHLD'} = \&Reaper;
unless ( $pid = fork ) {
    &runCommand();
    exit;
}


sub runCommand
{
    my ($stdout, $stderr, $success, $exit_code) = 
        capture_exec('perl', '-e', 'print "Test"');
    print "stdout: $stdout\n";
    print "stderr: $stderr\n";
    print "Success: $success\n";
    print "exit code: $exit_code\n";    
}

sub Reaper {
    my $pid;
    waitpid(-1, &WNOHANG);
}

killall('HUP', -$$);
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Update: added a kill.