I want to use a BEGIN block to set a global context variable that other modules will use to adjust their control flow. But I remember the C++ "static initialization" pattern, which ran code before 'main' was called.

The question is
-When are initializations done?
-Are initialization code waiting to execute when the block is first entered?
-Are File globals initialized at parse or execute time?
-Is there any difference when initializing from a constant? a variable? a variable that is initialized from a constant? a function call?

When I began this 2 years ago, I could find no good documentation on the pragma "vars" other than "it pre-allocates global variables". That really says nothing about scope, mechanics, accessibility,... But I did get 'our' to work, and I like the explicit "Gbl::" prefix as self documentation.

This is the module of global values.

#GBL.pm
package Gbl;
#== Context ============
our $runHTTP = 1;
our $runSMTP = 2;
our $runContext = $runHTTP;  # HTTP ...variable assignment might be af
+ter BEGIN
#our $runContext = 1;         # HTTP ...variable assignment might be a
+fter BEGIN
# ...
1
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This is the main controlling file (which will 'use Gbl;"

#main.pl
use feature qw( :5.10 );
use strict;
use warnings;
use File::Temp qw/ tempfile tempdir /;
use Email::MIME;
use lib "/usr/local/mySystem";
use Gbl;
BEGIN{
  # Set some Globals for this environment
  $Gbl::runContext = $Gbl::runSMTP;  
  #$Gbl::runContext = 2;  
  $Gbl::pi_LogPath = "/tmp/primarylog.log" ;  # log 
  $Gbl::ds_LogPath = "/tmp/datalog.log" ;
}  
use parseHTMP;
use parseSMTP;
#...
[download]

If the initialization executes immediately after parse, then

1. $runHTTP is set to 1 durnig Gbl parse upon 'use' in main.pl
   
2. $runSMTP is set to 2 durnig Gbl parse upon 'use' in main.pl
   
3. $runContext = $runHTTP upon parse (==1)
4. $Gbl::runContext = $Gbl::runSMTP (==2) upon BEGIN in main.pl

1. code to set $runHTTP to 1 is prepared durnig Gbl parse upon 'use' in main.pl
   
2. code to set $runSMTP to 2 is prepared durnig Gbl parse upon 'use' in main.pl
   
3. code to set $runContext = $runHTTP is prepared upon parse
4. $Gbl::runContext = $Gbl::runSMTP (== undef) upon BEGIN in main.pl
5. Execution finally passed through Gbl, causing initialization code to execute.
   $Gbl::runContext = $Gbl::runHTTP

The initializers had not yet run in Gbl to define the variables at the point when main.pl's BEGIN block runs and accesses them?