That's an odd one. I've reduced your example to a single file test case...

use strict;
use warnings FATAL => qw(all);
use Test::More tests => 1;

BEGIN {
    package Wah;
    $INC{'Wah.pm'} = __FILE__;
    sub import {
        my $caller = caller;
        ::is($caller, "Foo");
    }
}

BEGIN { *main::Foo:: }

package Foo;
use Wah;
[download]

G'day ainkov,

Welcome to the monastery.

I investigated this for a while. I have some findings but, as yet, no definitive answer. I don't have time to continue this now. I'll post what I have: that may help you, or someone else, towards a resolution; if not, I can continue to look into this tomorrow.

I created MyModule.pm and Importer.pm exactly as you posted them. They remained unchanged throughout all tests. All tests below used modified versions of test.pl.

I initially looked at the %main::MyModule:: stash (without referencing the $main::MyModule::{HASH} key):

$ cat test.pl
#!/usr/bin/env perl
use strict;
use warnings;
use Data::Dumper;
require 'MyModule.pm';
print Dumper \%main::MyModule::;

$ test.pl
main::MyModule
$VAR1 = {
          'BEGIN' => *::MyModule::BEGIN
        };
[download]

So, "main::MyModule", in the output, is not dependent on "my $symtab = *{'main::MyModule::'}{HASH};"; %main::MyModule:: is created with the key $main::MyModule::{BEGIN}. Next I tried the require within an explicit BEGIN block:

$ cat test.pl
#!/usr/bin/env perl
use strict;
use warnings;
use Data::Dumper;
BEGIN {
    require 'MyModule.pm';
}
print Dumper \%main::MyModule::;

$ test.pl
MyModule
$VAR1 = {
          'BEGIN' => *MyModule::BEGIN
        };
[download]

Now we get "MyModule" instead of "main::MyModule"; %main::MyModule::, with just the key $main::MyModule::{BEGIN}, is the same. I repeated both of those tests with "require MyModule;". Without a BEGIN block:

$ cat test.pl
#!/usr/bin/env perl
use strict;
use warnings;
use Data::Dumper;
require MyModule;
print Dumper \%main::MyModule::;

$ test.pl
MyModule
$VAR1 = {
          'BEGIN' => *MyModule::BEGIN
        };
[download]

And with a BEGIN block:

$ cat test.pl
#!/usr/bin/env perl
use strict;
use warnings;
use Data::Dumper;
BEGIN {
    require MyModule;
}
print Dumper \%main::MyModule::;

$ test.pl
MyModule
$VAR1 = {
          'BEGIN' => *MyModule::BEGIN
        };
[download]

In both of those cases, we have "MyModule" instead of "main::MyModule"; and %main::MyModule::, with just the key $main::MyModule::{BEGIN}, is still present and unchanged. I next looked at "use MyModule;":

$ cat test.pl
#!/usr/bin/env perl
use strict;
use warnings;
use Data::Dumper;
use MyModule;
print Dumper \%main::MyModule::;

$ test.pl
MyModule
$VAR1 = {
          'BEGIN' => *MyModule::BEGIN,
          'import' => *MyModule::import
        };
[download]

Now, %main::MyModule:: has an additional key: $main::MyModule::{import}.

That's where I'll have to leave it for now.