I came across some code that I'm trying to understand. It involves the symbol table. I read about the symbol table in a post Of Symbol Tables and Globs, but I have questions I couldn't find the answers to there. 

use Data::Dumper;

 $hash = {
     'user1' => {
             'oldpassword' => 0,
             'filesize' => '14360',
             'logins' => 1
      },
      'user2' => {
             'oldpassword' => 0,
             'filesize' => '1220',
             'logins' => 15
      },
      'user3' => {
             'oldpassword' => 1,
             'filesize' => '1780',
             'logins' => 7
      }
};

sub KeysByLogins {
  my $hash = shift;
  map { $_->[1] }
  sort { $a->[0] <=> $b->[0] }
  map { [ $hash{$_}->{logins}, $_ ] } keys %$hash;
}

our @keys = KeysByLogins($hash);
foreach my $key (@keys) {
  print Data::Dumper->Dump([$hash->{$key}], [$key]) . "\n"; # number 1
  #print Data::Dumper->Dump([$hash{$key}], [$key]) . "\n";    # number
+ 2
}

print  $::{hash}. ' ', $::{keys};

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The output was: 
$user1 = {
           'filesize' => '14360',
           'oldpassword' => 0,
           'logins' => 1
         };

$user3 = {
           'filesize' => '1780',
           'oldpassword' => 1,
           'logins' => 7
         };

$user2 = {
           'filesize' => '1220',
           'oldpassword' => 0,
           'logins' => 15
         };

*main::hash *main::keys

[download]
The line inside the sub map { [ $hash{$_}->{logins}, $_ ] } keys %$hash; isn't using the reference notation $hash->{logins} but is referring to the hash %hash, which wasn't declared. I'm guessing it springs up from the glob(?) in the symbol table (for the duration of the sub).

Then, later on in the print loop, the hash ref is used in line no. 1.

If this line is commented out and line 2 is uncommented, Dumper shows the hash as empty (when accessing the undeclared %hash variable).

Would that be because %hash only springs up in the scope of the subroutine, then is inaccessible outside the sub (in the print routine).

I guess I don't understand the workings of the symbol table. Thanks for any clear explanation of this code snippet.

In reply to Question about the symbol table by Cristoforo

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