IPC::Open3 and output

[Mike Martin]

Hi
I am trying to use Open3 to capture stderr and avoiding the shell ie:
my $str="-v 35 -i /data/Downloads/testinput.mp4 -c:v libx264 -preset fast
-crf 28 -g 25  -c:a  libmp3lame -b:a 128000  -threads 4 -af volume=2.5  -vf
scale='352:trunc(ow/((1/sar)*dar)/2)*2',fps='fps= 20',setsar='1/1'
-profile:a aac_he_v2 -strict -2 -y /home/mike/testopen.mp4";
  use Text::ParseWords;
@args = quotewords('\s+', "'", $str);

use IPC::Open3;

local(*HIS_IN, *HIS_OUT, *ERR);
my $cmd='ffmpeg';
my $pid=open3(undef, undef, *ERR,$cmd, @args) or die print "$!";
my $line;

while (sysread ERR, $line, 256){
print $line;
}
waitpid($pid, 0);

However the output is buffered for around 2 minutes at a time rather
immediately being printed

Is there any way around this