Here's a one line perl program that does nothing but set up a variable ... except that it does a lot!

$p[7]{john}{paul}[9] = 1;
 
__END__
 
A list called @p. Elements 0 to 6 are null, and element 7
is a reference to a hash ...
 
That hash contains a single member, key is "john" and the
values it holds is a reference to another hash ...
 
THAT hash contains a single member with a key of "paul" and
it contains a reference to a list ...
 
And that list has elements 0 to 8 being null, and element
nine containing the value 1.
 
AND THAT WAS ALL DONE AUTOMATICALLY by Perl's Autovivification
capability!!

If you assign to a variable in Perl and it does not already exist, the Perl creates it dynamically - and that implies that it also creates all the other necessary structures as well. My oneliner above took 10 lines to describe in plain English!

This is both powerful and dangerous. Something as simple as

$phone[1225708225] = 1;

will give you "out of memory" as a list of over one billion elements is calledup. But do remember that

$phone{1225708225} = 1;

will create a hash with just one member, and THAT is what you should use for data with sparsely distributed numeric keys
(written 2008-01-21)

 P217 - Perl - More than Simple Lists and Hashes!
  [43] Hash of lists in Perl - (2004-09-09)
  [293] Course follow-ups - (2005-04-27)
  [2241] Perl references - $$var and \$var notations - (2009-06-15)
  [2840] Just pass a pointer - do not duplicate the data - (2010-06-30)
  [2877] Further more advanced Perl examples - (2010-07-19)
  [2996] Copying - duplicating data, or just adding a name? Perl and Python compared - (2010-10-12)
  [3007] Setting up a matrix of data (2D array) for processing in your program - (2010-10-21)
  [3072] Finding elements common to many lists / arrays - (2010-11-26)
  [3105] Adventure with references to lists and lists of references - (2010-12-26)
  [3118] Arrays of arrays - or 2D arrays. How to program tables. - (2011-01-02)
  [3399] From fish, loaves and apples to money, plastic cards and BACS (Perl references explained) - (2011-08-20)
  [3406] Not multidimentional arrays - but lists of lists. Much more flexible. Perl! - (2011-08-26)
  [3444] Take the dog on a lead - do not carry her. Perl references. - (2011-09-17)
  [3577] How to do multidimensional arrays (or rather lists and hashes) in Perl - (2012-01-14)
  [3906] Taking the lead, not the dog, for a walk. - (2012-10-28)

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Downloading data for use in Excel (from PHP / MySQL)
Python - formatting objects
Pictures you can use - for free - from our library
Keeping staff up to date on hotel room status
Autovivification - the magic appearance of variables in Perl
Perl, PHP or Python? No - Perl AND PHP AND Python!
Summer Ball at Bowood - Saturday 12th July 2008
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Handling Binary data (.gif file example) in Perl
Extracting information from a file of records