#! perl -sw
use strict;
use threads;
use IO::Select;

$|++;

my $sel = IO::Select->new();
my %children;

for (1..2) {
    open my $fh, q[
        perl -le'$|++; sleep(1), print qq[$$: $_] for 1 .. 10' |
    ] or die $!;

    $sel->add($fh);
    $children{$fh} = "";
}

while (%children) {
    for my $fh ( $sel->can_read() ) {
        our $buf; local *buf = \( $children{$fh} );  # alias

        sysread($fh, $buf, 4096, length($buf))
            or do { print("$buf\n") if length($buf);
                    $sel->remove($fh);
                    delete($children{$fh});
                    next;
                  };

        print $1 while $buf =~ s/^(.*\n)//;
    }
}
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Definitely more complex and less scalable.