So I want a logger object, and only one logger object thought it may be used in various packages. I follow the simple instructions at https://www.perl.com/article/52/2013/12/11/Implementing-the-singleton-pattern-in-Perl but when I test it I see I get two objects. In the following test one gets its 'Log' value altered and the other does not ...

The test ...

perl -MData::Dumper -Mlib=. -MHelper::Lgr -E'     \
    $l=Helper::Lgr->getInstance();$l->log(0,"Log1"); \
    $z=Helper::Lgr->getInstance();$l->set('Log', 4); \
    print Dumper $l; \
    print Dumper $z'

Get instance called from main
Not yet instantiated
Sun Dec 29 18:51:08 2019 Log1
Get instance called from main
Not yet instantiated
$VAR1 = bless( {
                 'LogName' => 'CommandLine.log',
                 '_continuation' => 0,
                 'Screen' => 0,
                 'LogDir' => '/home/random/log',
                 'fh' => \*Helper::Lgr::__ANONIO__,
                 'Log' => 4
               }, 'Helper::Lgr' );
$VAR1 = bless( {
                 'LogDir' => '/home/random/log',
                 'Screen' => 0,
                 'LogName' => 'CommandLine.log',
                 '_continuation' => 0,
                 'Log' => 0,
                 'fh' => \*Helper::Lgr::__ANONIO__
               }, 'Helper::Lgr' );
[download]

Here is the start of the Logger pm that I am using, the set method is pretty much what you expect,so not reprduced here ...

#!/usr/bin/perl
#
# Lgr.pm
#
# Controls both logging to a file and to the screen
# you can set a verbosity for each independently
#
package Helper::Lgr;
use strict;
use warnings;
use v5.10.0;
use File::Basename;   
use File::Path qw(make_path);
use FindBin;
use Data::Dumper;
my $script = basename( $0, '.pl', '.pm' );

# Logger is a classic singleton pattern. We only want one instance in 
+our code
my $instance = undef; 

sub getInstance {
    say "Get instance called from ".( caller );
    if ($instance) { # Already instantiated, check if we need to chang
+e any settings
        say  "Already instantiated";
        return $instance;
    }
    else {
        say  "Not yet instantiated";
    }
    my $class    = shift;
    my $instance = shift;  # settings to use
    $instance = {} unless $instance;
    die "$instance is not a hash ref\n" unless ref $instance eq 'HASH'
+;
    bless $instance, $class; # we bless early so we can call methods o
+n ourself

    ... And much more ...
[download]

Cheers,
R.