#! perl -slw
use strict;
use threads qw[ yield async ];
use threads::shared;

my( $cmd, $file ) = @ARGV;
my $done : shared = 0;
my @lines : shared;

async {
	my $pid = open my $CMD, "$cmd |" or die "$cmd : $!";
	open my $fh, '>', $file or die "$file : $!";
	while( <$CMD> ) {
		chomp;
		print $fh $_; 		## output to the file
		push @lines, $_;	## and push it to a shared array
	}
	$done = 1;
}->detach;

my $n = 0;
while( !$done ) {
	if( @lines ) {			## lines to be processed
		print pop @lines;   ## process them
	}
	else {
		## Else nothing to do but wait.
		yield;
	}
}