my $pid = open my $capture, 'ffmpeg -r 15 -f vfwcap -i 0 -f rgba -o - |'
    or die "$! / $?";
binmode $capture;
# assume rgba
my $frame_size = $width * $height * 4;
$/ = \$frame_size;

while (my $frame = <$capture>) {
    ...
};