How to change the input separator when we read a word file using perl?

By assigning to $/.

But if you mean "MS Office Word .doc file" when you say "word file", that won't help you much, because those are binary files, and do not use a fixed record separator. For reading binary files, you should use binmode and read. Or a CPAN module which does all the hard work for you. Or don't use .doc files in the first place.