@new = ('Al');
while(@new){
 $key = shift @new;
 push @old, $key;
 @friends = (select col1 from table where col2 = $key,
             select col2 from table where col1 = $key);
 %unique = ();
 @unique{@friends} = undef;
 delete @unique{@new, @old};
 push @new, keys %unique;
}