Well, first off, $_ and @_ have absolutely no relationship with one another. None. So, even if you only pass one item, $_ isn't set to it. (Unless, of course, you do something like $_ = shift;)

Now, you are correct that $_ is "remembering" when it was last set. This is because $_ (and @_) are both global variables in the truest sense of the word. (This, btw, is why you should always do local $_; in a sub within a module you intend on manipulating $_ within.)

So, the question I ask you is why aren't you doing something like

sub parse{
    my $self=shift;
    my ($line) = @_;

    ($self->{'AN'}) = unpack("A5", $line);

    print $self->{'AN'}."\n";
}
[download]

Or, some other, more relevant, variable name ...

------
We are the carpenters and bricklayers of the Information Age.

The idea is a little like C++ templates, except not quite so brain-meltingly complicated. -- TheDamian, Exegesis 6

Please remember that I'm crufty and crochety. All opinions are purely mine and all code is untested, unless otherwise specified.