Let's say that BitStream inherits from Stream::Generic. If you call BitStream's new() your way, you cannot do my $self = $class->SUPER::new(@_); to have some intialization deferred to the parent. Better is my $stream = BitStream->new("bigbinfile"); - much easier to extend, now. Plus, you could do something like:

# This way of dealing with meg and gig is a poor way, used only for de
+monstration.
# Supersearch for a better way.
my $KB = 1024;
my $MB = 1024 * $KB;
my $GB = 1024 * $MB;
my %Classes = (
    $MB => 'BitStream::Vec',
    $GB => 'BitStream::Buffered',
);

my $filesize = -s $filename;

my $classname = 'BitStream::InMemory';
foreach my $min_size (sort { $a <=> $b } keys %Classes)
{
    last unless $filesize >= $min_size;
    $classname = $classes{$min_size};
}

my $stream = $classname->new($filename);
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That way, you can choose your BitStream::* class based on the size of your file. If it's under a meg, use the instream. Between a meg and a gig, use the hybrid vec option. Over a gig, you need to use the slow buffered method.