I can see that being difficult, as I've read in a few places that pipe opens aren't supported in Perl threads.

Hm. Where? I've never heard of, nor encountered, any problems with piped-opens in threads. (unless its a *nix thing I've never seen reported.)

Try this which will happily run tens of concurrent piped-open commands on Windows:

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

our $T //= 10;
our $N //= 1000;

my $Q = new Thread::Queue;

async{
    my $tid = threads->tid;

    open my $cmd, '-|', 'count.pl ' . int( rand $N ) or die $!;

    $Q->enqueue( "$tid:$_" ) while defined ( $_ = <$cmd> );
    $Q->enqueue( "$tid:done\n" );
    $Q->enqueue( undef );
}->detach for 1 .. $T;

for( 1 .. $T ) {
    while( $_ = $Q->dequeue ) {
        printf "%s", $_;
    }
}

__END__
##This is count.pl
#! perl -slw
use strict;

$|++;
my $i = $ARGV[0] // 30;

print --$i while $i and sleep 1;
[download]

Eg

C:\test>t-popen -T=10 -N=10
8:done
1:2
2:4
4:5
3:5
6:4
5:0
5:done
7:0
7:done
9:8
10:4
1:1
2:3
4:4
3:4
6:3
9:7
10:3
1:0
1:done
2:2
3:3
4:3
6:2
9:6
10:2
2:1
4:2
3:2
6:1
9:5
10:1
2:0
2:done
4:1
3:1
6:0
6:done
9:4
10:0
10:done
4:0
3:0
4:done
3:done
9:3
9:2
9:1
9:0
9:done
[download]