This will start four processes on my 4 core system within 0.2 of a second of each other:

#! perl -sw
use strict;
use threads;
use threads::shared;
use Thread::Queue;
use Time::HiRes qw[ time ];

our $T //= 4; ## 80

my $sem :shared = 0;

my @commands = map qq[perl -MTime::HiRes=time -le"print \$\$, ':', tim
+e()"], 1 .. $T;

sub doit {
    my( $cmd, $Q ) = @_;
    lock $sem;
    my $pid = open my $pipe, '-|', $cmd or die $!;
    $Q->enqueue( "$pid:$_" ) while <$pipe>;
    $Q->enqueue( undef );
}

my $Q = new Thread::Queue;
{
    lock $sem;
    async( \&doit, $_, $Q )->detach for @commands;
    sleep 2;
}

open OUT, '>', 'log' or die $!;

for( 1 .. $T ) {
    print OUT while defined( $_ = $Q->dequeue );
}

close OUT;

__END__
c:\test>type log
25692:25692:1481831121.12942
26080:26080:1481831121.18679
26088:26088:1481831121.2571
25704:25704:1481831121.32448
[download]

To get closer than that you'd need to move the semaphoring into the child processes.