Problem

You want your variables to retain their values between calls to your program.

Solution

Use a MLDBM to store the values between calls to your program:

use MLDBM 'DB_File';  my ($VARIABLE1,$VARIABLE2); my $Persistent_Store = '/projects/foo/data'; BEGIN {     my %data;     tie(%data, 'MLDBM', $Persistent_Store)         or die "Can't tie to $Persistent_Store : $!";     $VARIABLE1 = $data{VARIABLE1};     $VARIABLE2 = $data{VARIABLE2};     # ...     untie %data; } END {     my %data;     tie (%data, 'MLDBM', $Persistent_Store)         or die "Can't tie to $Persistent_Store : $!";     $data{VARIABLE1} = $VARIABLE1;     $data{VARIABLE2} = $VARIABLE2;     # ...     untie %data; }

Discussion

An important limitation of MLDBM is that you can't add to or alter the structure in the reference without assignment to a temporary variable. We do this in the sample program in Example 14.6 , assigning to $array_ref before we push . You simply can't do this:

push(@{$db{$user}}, $duration);

For a start, MLDBM doesn't allow it. Also, $db{$user} might not be in the database (the array reference isn't automatically created as it would be if %db weren't tied to a DBM file). This is why we test exists $db{$user} when we give $array_ref its initial value. We're creating the empty array for the case where it doesn't already exist.

#!/usr/bin/perl -w # 

mldbm_demo - show how to use MLDBM with DB_File  use MLDBM "DB_File";  $db = "/tmp/mldbm-array";  tie %db, 'MLDBM', $db   or die "Can't open $db : $!";  while(<DATA>) {     chomp;     ($user, $duration) = split(/\s+/, $_);     $array_ref = exists $db{$user} ? $db{$user} : [];     push(@$array_ref, $duration);     $db{$user} = $array_ref; }  foreach $user (sort keys %db) {     print "$user: ";     $total = 0;     foreach $duration (@{ $db{$user} }) {         print "$duration ";         $total += $duration;     }         print "($total)\n";     }  __END__ gnat        15.3 tchrist     2.5 jules       22.1 tchrist     15.9 gnat        8.7

Newer versions of MLDBM allow you to select not just the database module (we recommend DB_File), but also the serialization module (we recommend Storable). Previous versions limited you to Data::Dumper for serializing, which is slower than Storable. Here's how you use DB_File with Storable:

use MLDBM qw(DB_File Storable);

See Also

The documentation for the Data::Dumper, MLDBM, and Storable modules from CPAN; Recipe 11.13 ; Recipe 14.8