Notes

1. On some operating systems (e.g., UNIX), you can make a script executable. The operating system then uses the first line in the script to find the interpreter. On other operating systems (like MS-Windows), the Perl interpreter has to be called explicitely, and the script is given as a command line parameter to Perl, e.g., c:\perl myscript.pl. On the Mac, it's a different story again. It is always a good idea to include this line, even on operating systems that cannot execute scripts directly.
2. The syntax for the Perl function print is
   print FILEHANDLE LIST
   and prints a string or comma separated list of strings to the file designated by FILEHANDLE. If FILEHANDLE is omitted, then the function prints to the currently selected output filehandle, initially STDOUT[Note 4].
3. $_ is a Global Special Variable. It's the default input and pattern matching space, and assumed by Perl in several places, even if you don't specify it.
4. STDOUT is a Global Special Filehandle. The others are: ARGV, STDERR, STDIN, DATA, and _.
5. A hash is a data type that contains key-value pairs, e.g.
   $myHash{"TCP"} = "Transmission Control Protocol";
   $myHash{"IP"} = "Internet Protocol";
   Note the curly braces {}. The variable name prefix for the whole hash is %, used, e.g., when initializing the hash to an empty list
   %myHash = ()
   while the variable name prefix when accessing a single element is $.