You don't show what you've tried so far, so I'll give you a general description of how I'd approach the problem. The problem can be split up into three parts:

1. Loop through each line of the file
2. Check whether the current line matches your target ("starts with 'Article'")
3. Output the reformatted line

The code for each part also is relatively self-contained:

1. #!perl -w
   use strict;
   
   # loop through the file
   while (<>) {
       print
   };
   [download]

2. if (/^Article\b/) {
       s/\s*$//; # remove trailing whitespace and newlines
       print "Line [$_] is a heading.";
   };
   [download]

3. $_ = "Article 1 News";
   print "\\subsection*{$_}";
   [download]

You'll have to assemble these parts into a coherent whole and/or ask questions about the parts that are still unclear to you. If you chose a different approach, it will help us to help you better if you show your approach.