Wups, mybad on the terminology. I didn't know what to call them but global doesn't seem right either. *shrug* As far as these variables go, my limits the scope in such a way that in a different module the variable isn't available. Try separating the packages into their own modules, like in this example:

Mytest.pm: 

package Mytest;
use strict;
use warnings;

my $var = 1;

1;

[download]
And then a user of the module: 
use strict;
use warnings;
use Mytest;

print $Mytest::var . "\n";

[download]
You'll get this: 
Name "Mytest::var" used only once: possible typo at usemytest.pl line 5.
Use of uninitialized value in concatenation (.) or string at usemytest.pl line 5.
Or this: 
use strict;
use warnings;
use Mytest;

print $var;

[download]
You'll get this: 
Global symbol "$var" requires explicit package name at usemytest.pl line 5.
Execution of usemytest.pl aborted due to compilation errors.
Without strict and warnings: 
use Mytest;
print $var;

[download]
or 
use Mytest;
print $Mytest::var;

[download]
You get nothing; $var is completely invisible outside it's module...or am I missing something? :/ Is there another way to access that scalar that I'm missing?

As far as the example goes, it is an overly simplified demonstration of some interactions between many modules in a medium sized tool I'm working on. Here's a slightly more concrete usage, and two instances where the issue happens. p1 shows the same kind of issue as before: by assigning $undef to the variable on the same line it's created, if used later in a begin block, then the undef overrides anything done during the begin block. p2 shows the issue in a much more real world scenario: you have a simple inside out class, and decide to 'initialize' the "private variable" hashes. For that example, I'm trying it three different ways: initializing in line, using the default 'my' behavior, and initializing in a begin block, to show what happens in each of these scenerios: 

#!/usr/bin/perl -w

package Problem;

use strict;
use warnings;
use Scalar::Util qw(refaddr);

{
   my %key1_of;      # leave blank test
   my %key2_of = (); # initialize test
   my %key3_of;
   BEGIN
   {
      %key3_of = (); # initialize with a BEGIN
   }

   sub new
   {
      my ($class, $ar1, $ar2, $ar3) = @_;
      my $new_object = bless \do{my $anon_scalar}, $class;
      $key1_of{refaddr($new_object)} = $ar1;
      $key2_of{refaddr($new_object)} = $ar2;
      $key3_of{refaddr($new_object)} = $ar3;
      return $new_object;
   }

   sub get_key1
   {
      my $self = shift;
      return $key1_of{refaddr($self)};
   }

   sub get_key2
   {
      my $self = shift;
      return $key2_of{refaddr($self)};
   }

   sub get_key3
   {
      my $self = shift;
      return $key3_of{refaddr($self)};
   }

}

1;

# Here's a user of the above object.

package Main;  


use strict;
use warnings;

# Below: Creating values for the sake of showing the issue at hand...

# First variable: assigning undef (declaring its initial value), 
# but due to a subroutine or other complex series of processing, it
# gets assigned a value during a BEGIN block
my $p1 = undef;   # using the 'always assign a value' paradigm here
my $p2;           # leaving it blank for now
my $p3 = undef;   # same 'always assign' paradigm here, but not used i
+n 'begin'

# some processing happens in here; for example, let's say these variab
+les are
# dependent on user input or automatic config files, so $p1, $p2, $p3 
+might 
# not ever be used.  It just turns out that this time, they all are.

# However, the user of the package creates these instances of the obje
+ct in a
# begin block, and for whatever reason, the user can't avoid doing thi
+s.
BEGIN
{
   $p1 = Problem->new('P11', 'P12', 'P13');
   $p2 = Problem->new('P21', 'P22', 'P23');
}
$p3 = Problem->new('P31', 'P32', 'P33');

sub printit # for display purposes
{
   my $p = shift;
   if (defined $p)
   {
      print "   k1: " . $p->get_key1() . "\n";
      my $k2 = $p->get_key2();
      $k2 = "(undef)" if ! defined $k2;
      print "   k2: $k2\n";
      print "   k3: " . $p->get_key3() . "\n";
   }
   else
   {
      print "   Error: Object is undefined.\n";
   }
};


print "P1:\n";
printit($p1);
print "   (Due to scoping of the undef assignment, the object itself i
+s lost.)\n";
print "P2:\n";
printit($p2);
print "   (Note loss of key 2's value; it's lost because %key2_of\n" .
      "    is assigned to () after the object's creation).\n";
print "P3:\n";
printit($p3);
print "   (As expected, no issues at all).\n";

[download]
And then, the output is: 
P1:
   Error: Object is undefined.
   (Due to scoping of the undef assignment, the object itself is lost.)
P2:
   k1: P21

   k2: (undef)
   k3: P23
   (Note loss of key 2's value; it's lost because %key2_of
    is assigned to () after the object's creation).
P3:
   k1: P31
   k2: P32
   k3: P33
   (As expected, no issues at all).
As you can see, p1 is completely wiped out by the initial assignment and p2 loses it's member variable due to the assignment of () to it after the object is created in the BEGIN block. Setting up the variable in a begin block ie, initializing it to 'empty', doesn't cause any problems for users, no matter where they use it.

The issue here is if we're initializing a value, and the initial value is empty or undef, why do it twice? It simply makes the initialization process even more complex. If we're not using it or not using it right away, we might as well take advantage of the behavior for perl to automatically setting scalars to undef or arrays/hashes to empty.

Or am I just completely off about all this?

In reply to Re^3: Use of uninitialized variables? by Glav
in thread Use of uninitialized variables? by Zadeh

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.