#!/usr/bin/perl

use strict;

my $tossMeAbout = 'Initial value';

{
    my $updatedValue = &adjustValue($tossMeAbout);
    print "-------------------------------------------------------------------------------\n";
    print "    MAIN:       \$tossMeAbout = [$tossMeAbout]\n";
    print "    MAIN:      \$updatedValue = [$updatedValue]\n";

    my $additionalValue = &adjustValue($updatedValue);
    print "-------------------------------------------------------------------------------\n";
    print "    MAIN:       \$tossMeAbout = [$tossMeAbout]\n";
    print "    MAIN:      \$updatedValue = [$updatedValue]\n";
    print "    MAIN:   \$additionalValue = [$additionalValue]\n";

    print "-------------------------------------------------------------------------------\n";
}

exit;

{
    my $persistentValue;

    sub adjustValue
    {
        my ($requestedValue, @garbage) = @_;
        if (!defined $requestedValue) { $requestedValue = ''; }

        my $returnValue = '';

        print "-------------------------------------------------------------------------------\n";
        print "SUB BEGIN:   \$requestedValue = [$requestedValue]\n";
        print "SUB BEGIN:  \$persistentValue = [$persistentValue]\n";
        print "SUB BEGIN:      \$returnValue = [$returnValue]\n";

        # Append requested value to persistent value
        if (!defined $persistentValue)
        {
            $persistentValue = $requestedValue;
        }
        else
        {
            $persistentValue .= "/$requestedValue";
        }

        $returnValue = "<$requestedValue>";

        print "-------------------------------------------------------------------------------\n";
        print "  SUB END:   \$requestedValue = [$requestedValue]\n";
        print "  SUB END:  \$persistentValue = [$persistentValue]\n";
        print "  SUB END:      \$returnValue = [$returnValue]\n";

        return $returnValue;
    }
}

__END__

##</code><code>##

C:\Steve\Dev\PerlMonks\P-2015-03-19@1121-Durable-Subroutine-Data>persistent1.pl
-------------------------------------------------------------------------------
SUB BEGIN:   $requestedValue = [Initial value]
SUB BEGIN:  $persistentValue = []
SUB BEGIN:      $returnValue = []
-------------------------------------------------------------------------------
  SUB END:   $requestedValue = [Initial value]
  SUB END:  $persistentValue = [Initial value]
  SUB END:      $returnValue = [<Initial value>]
-------------------------------------------------------------------------------
     MAIN:       $tossMeAbout = [Initial value]
     MAIN:      $updatedValue = [<Initial value>]
-------------------------------------------------------------------------------
SUB BEGIN:   $requestedValue = [<Initial value>]
SUB BEGIN:  $persistentValue = [Initial value]
SUB BEGIN:      $returnValue = []
-------------------------------------------------------------------------------
  SUB END:   $requestedValue = [<Initial value>]
  SUB END:  $persistentValue = [Initial value/<Initial value>]
  SUB END:      $returnValue = [<<Initial value>>]
-------------------------------------------------------------------------------
     MAIN:       $tossMeAbout = [Initial value]
     MAIN:      $updatedValue = [<Initial value>]
     MAIN:   $additionalValue = [<<Initial value>>]
-------------------------------------------------------------------------------