Thanks for your quick reply. I need case 2. As a teacher, I want to find out to what extent any two students in my class have copied each other's assignment. Majority of the students (out of 30) are sincere and hard working. But it appears to me that nearly four students often plagiarize their written assignments i.e. I think they copy from others' assignments without visiting library or consulting textbooks/research papers. That is why I need a working perl script which can detect the degree of plagiarism adopted by the doubtful students. This is a very personal case. I just want to tell the students that I am not satisfied with their assignments should I detect more than 80% matched words. I don't know whether perl script can solve this problem faced by me. I want to make those (four) students more hard-working not only in studies but also in other spheres of life.