The language is Perl, and the interpreter program is perl. PERL is someone speaking too loudly because they don't know the language. You are now initiated into the inner circle. :-)

As a very general rule, post some code so that we can see what part of the program you are really having trouble with. If you've never written anything at all in Perl, you should probably start with a Tutorial introduction. There are scads of them available.

You should know how to read a file line by line, which is the usual approach in Perl, unless there's some reason to do something else. You'll then want to extract every word from a line, which is a basic regular expression task, depending on how you define a word. perldoc perlretut will give you ideas there.

For more specific advice on this task, you might find How do I count the frequency of words in a file and save them for later? in the Categorized Questions and Answers section to be helpful.