my $stream = BitStream::new("bigbinfile");

##</code><code>##

package File::Bitstream;
use strict;
use warnings;

sub new {
    my ($class,$file)=@_;
    open my $fh,"<",$file or die "Cant read '$file':$!";
    binmode $fh;
    return bless { file=>$file, fh=>$fh, buffer=>'', chunk=>1024 },$class;
}

sub get_bits {
    my ($self,$bits)=@_;
    while (!eof($self->{fh}) and length($self->{buffer})<$bits) {
        my $chars='';
        read($self->{fh},$chars,$self->{chunk});
        $self->{buffer}.=unpack "B*",$chars;
    }
    return length($self->{buffer}) ? substr($self->{buffer},0,$bits,'') : undef;
}

1;

my $o=File::Bitstream->new($0);
my $bits='';
print $bits,$/ while defined($bits=$o->get_bits(13));