my $file = 'c:/test.pl';
my $BLOCK_SIZE = 1024*1024;
open my $fh, $file or die $!;
END{close $fh}
my ( $buffer, $buf );
read( $fh, $buf, $BLOCK_SIZE );
$buffer .= unpack "B*", $buf;

sub get_bits {
    my ( $num_bits ) = @_; # faster than shift
    unless ( length($buffer) > $num_bits ) {
        read( $fh, $buf, $BLOCK_SIZE );
        $buffer .= unpack "B*", $buf;
        die "No more bits left" if length($buffer) < $num_bits;
    }
  return substr $buffer, 0, $num_bits, '';
}
    

for ( 1..1000 ) {
    print get_bits(16), $/;
}