Someone

I am trying to write a sub to split up a string of hexadecimal numbers into pairs of two hex digits, then print their 8-bit binary representation. Obviously, I don't understand how pack is supposed to work. Why isn't this printing out the proper binary representations of these numbers? 

<code>

sub bittify{

    my $string = pop;
    my @cols = ();    

    # Split the string into individual hex numbers 
    while (my $num = substr $string, 0, 2, ''){
	
	print $num, "\n";
        print unpack('b8', hex($num)), "\n";
    }
    
}

my $string1 = "001fffaa";
bittify($string1);

</code>