> what conditions must be in place in order to turn a 
    > complete program into a 'magic box' or function that
    > can be included in another program?
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Another issue is that the script you're require-ing has to return a true value; that's why such scripts often end with

    1;
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I'd recommend turning the function into a module. The easiest way to do this is to create a skeleton module using h2xs and then copy your code into the skeleton.

If you don't plan to reuse this code, just put it in, in-line. No packaging worries, nice and simple. Once you have the data structure, the rest of the code just diddles with it.

You could make it nicer by turning it into a subroutine, like this:

sub read_it_and_construct {
   # Arguments are in the @_ array here
   my ($filename) = @_;

   # Create variables for this function only using "my"
   my $DATA;

   # Open the file and read it
   open (INPUT, $filename) or die "Can't open $filename: $!\n";

   # ... your code here, which builds the structure ...
   # (don't forget to use 'my' on any other variables you
   # use in here! Otherwise, they're global and can clobber
   # like-named variables in the main program!)

   # Pass the structure back to the caller
   return $data;
}

my $structure = read_it_and_construct($from_this_filename);

If you want to make it really black-box, you can go object-oriented; I can talk to you about that if you like. Essentially, you write some more subroutines that know how to access the data structure, and return (to the caller) a special kind of reference that knows about these functions, so you can do stuff like

my $filetree = ReadAndConstruct->new($file);
my $value = $filetree->lookup($key1,$key2,$key3);

UPDATE:  

    based on the advice i read, i decided to simply 
    enclose it in a sub-function (thanks to all for 
    the advice)

    perhaps i could save that sub function in a file
    and add the line 'include myFunctions.pl' at the 
    beginning of the new document?

    i can deal with it the way it is, 
    (i've done it before, i can do it again ;)
    but it got me thinking... 

   what i really want is a good text-editor that has
   "block support" (along with regular colour and 
   tab support)

   the idea is that code could be displayed as a tree
   where blocks of it could be expanded and collapsed
   with the click of a toggle box

   ... navigation would be greatly improved (for 
   me at least) if after writing a complete and working 
   sub-function i could collapse it into one line 
   of code 

   i'll put it on my 'to do' list ... but that's under
   'learn how to program visual C++' so if someone else 
   knows of a text editor that does that already, 
   let me know  ;)
  
   (and pemungkah: you can bet that i'll be asking you 
      about coding in OO soon enough <g> )