use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);    

my $pid = open my $fh, '-|', 'sleep 3; echo aaaa' // die $!; 

my $flags = fcntl($fh, F_GETFL, 0) or die "failed to get flags, $!";      
fcntl($fh, F_SETFL, $flags | O_NONBLOCK) or die "Couldn't set file flags: $!\n";
my $output = <$fh>;
if( defined $output ){
        print "got output: '$output'\n"    
} else { print "no output right now.\n" }

print "now waiting to end ...\n";
#waitpid $pid, 0;
close($fh); 
#kill 'TERM', $pid;
print "end.\n";