That's a way of conveying a "usage synopsis" to a user when the command is run without appropriate arguments. In this case, the "synopsis" is a message that says the user should enter the name of the command (i.e. the name of the perl script) followed by the name of a stopword list file, followed by the name of a document text file.

The part between parentheses tests whether @ARGV contains two elements, and then whether each element is the name of an existing data file. If any of those three conditions is false, it goes into the "or die ..." clause, and the program exits with the usage synopsis.

In case your shell environment requires that you run "perl.exe" followed by the name of your script, just add the names of the two data files after the name of the script in order to get it to actually run with those two files as input. Don't forget to redirect STDOUT to a file:

perl  name_of_script.pl  stopword.file  doc.file > table.file
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