Have a look at this Perl program:

use fyle;
tie $counter,"fyle";
 
$counter = $counter + 1;
print ("This is access no. $counter\n");

Apart from the rather curious module loaded at the top, this seems to take an undefined variable, set it to one, and print it out. What a - err - pointless (!) program. But:

Dorothy-2:pl grahamellis$ perl bowtie
This is access no. 11
Dorothy-2:pl grahamellis$ perl bowtie
This is access no. 12
Dorothy-2:pl grahamellis$ perl bowtie
This is access no. 13
Dorothy-2:pl grahamellis$ perl bowtie
This is access no. 14
Dorothy-2:pl grahamellis$

What is happening?

Deep within Perl, there are a very limited number of operations that can be performed with a scalar - it can be created and destroyed, it can be read and written ... and that's about it. And these operations can be redefined by a class, and then applied to individual variables via a tie command ... which is what I have done here.

So in my example, the variable is persistent. Inside the class, it's actually stored in a data file that is read each time the variable is read, and overwritten each time a new values is saved to the variable. So it lasts from one run of the program to the next.

The source code shown above is also available here, and the definition of tieing a variable to a filecontents is here.

You can do a lot more with tieing. We have an example of connecting to a flat file database (NDBM) here (that calls up a standard tie class shipped with Perl). There's an example that forces strings in a variable to always be treated as lower case here and here. Finallly, a tie class which maps a file to a hash here.

---

Another new example / Perl trick ... see here for a short example / explanation of binary data handling in Perl
(written 2009-08-27, updated 2009-08-28)

 P304 - Perl - Tieing
  [2243] Changing a variable behaviour in Perl - tieing - (2009-06-16)
  [3007] Setting up a matrix of data (2D array) for processing in your program - (2010-10-21)
  [3409] When variables behave differently - Tie in Perl - (2011-08-28)

P212 - Perl - More on Character Strings
  [453] Commenting Perl regular expressions - (2005-09-30)
  [583] Remember to process blank lines - (2006-01-31)
  [586] Perl Regular Expressions - finding the position and length of the match - (2006-02-02)
  [597] Storing a regular expression in a perl variable - (2006-02-09)
  [608] Don't expose your regular expressions - (2006-02-15)
  [737] Coloured text in a terminal from Perl - (2006-05-29)
  [928] C++ and Perl - why did they do it THAT way? - (2006-11-16)
  [943] Matching within multiline strings, and ignoring case in regular expressions - (2006-11-25)
  [1222] Perl, the substitute operator s - (2007-06-08)
  [1230] Commenting a Perl Regular Expression - (2007-06-12)
  [1251] Substitute operator / modifiers in Perl - (2007-06-28)
  [1305] Regular expressions made easy - building from components - (2007-08-16)
  [1336] Ignore case in Regular Expression - (2007-09-08)
  [1510] Handling Binary data (.gif file example) in Perl - (2008-01-17)
  [1727] Equality and looks like tests - Perl - (2008-07-29)
  [1735] Finding words and work boundaries (MySQL, Perl, PHP) - (2008-08-03)
  [1947] Perl substitute - the e modifier - (2008-12-16)
  [2230] Running a piece of code is like drinking a pint of beer - (2009-06-11)
  [2657] Want to do a big batch edit? Nothing beats Perl! - (2010-03-01)
  [2801] Binary data handling with unpack in Perl - (2010-06-10)
  [2834] Teaching examples in Perl - third and final part - (2010-06-27)
  [2874] Unpacking a Perl string into a list - (2010-07-16)
  [2877] Further more advanced Perl examples - (2010-07-19)
  [2993] Arrays v Lists - what is the difference, why use one or the other - (2010-10-10)
  [3059] Object Orientation in an hour and other Perl Lectures - (2010-11-18)
  [3100] Looking ahead and behind in Regular Expressions - double matching - (2010-12-23)
  [3322] How much has Perl (and other languages) changed? - (2011-06-10)
  [3332] DNA to Amino Acid - a sample Perl script - (2011-06-24)
  [3411] Single and double quotes strings in Perl - what is the difference? - (2011-08-30)
  [3546] The difference between dot (a.k.a. full stop, period) and comma in Perl - (2011-12-09)
  [3630] Serialsing and unserialising data for storage and transfer in Perl - (2012-02-28)
  [3650] Possessive Regular Expression Matching - Perl, Objective C and some other languages - (2012-03-12)
  [3707] Converting codons via Amino Acids to Proteins in Perl - (2012-04-25)
  [3927] First match or all matches? Perl Regular Expressions - (2012-11-19)
  [4452] Binary data handling - Python and Perl - (2015-03-09)

Back to Handling XML in Perl - introduction and early examples

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Forward to Object Oriented programming - a practical design example

Lua Regular Expressions
Giving up on user input - keyboard timeout in Perl
Checking the database connection manually
Object Oriented programming - a practical design example
Making variables persistant, pretending a database is a variable and other Perl tricks
Handling XML in Perl - introduction and early examples
Wiltshire / Melksham Weddings - guest accommodation
Long job - progress bar techniques (Perl)
Designing your data structures for a robust Perl application
Lead characters on Perl variable names