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;
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