# example
@list1 = qw(a b c);
@list2 = qw(b c d);
# a b c 
#   b c d
#   1 2
list_overlap( \@list1, \@list2 );
# returns 2

@list1 = qw(a b c d b);
@list2 = qw(b c d);
# a b c d b
#         b c d
#         1
list_overlap(  \@list1, \@list2 );
# returns 1