T'is easier with threads:

#! perl -sw
use strict;
use threads;
use Thread::Queue;

$|++;

my $Q = new Thread::Queue;

async {
    open my $fh, q[
        perl -le"$|++; sleep(1), print qq[$$: $_] for 1 .. 100" |
    ] or die $!;
    $Q->enqueue( $_ ) while <$fh>;
    $Q->enqueue( undef );
}->detach;

async {
    open my $fh, q[
        perl -le"$|++; sleep(1), print qq[$$: $_] for 1 .. 100" |
    ] or die $!;
    $Q->enqueue( $_ ) while <$fh>;
    $Q->enqueue( undef );
}->detach;

for ( 1 .. 2 ) {
    print while defined( $_ = $Q->dequeue );
}
[download]