# ModuleA.pm

use ModuleB;

BEGIN {
    print "ModuleA::BEGIN\n";
}

1;

# ModuleB.pm
BEGIN {
    print "ModuleB::BEGIN\n";
}

1;

# main.pl

use ModuleA;
use ModuleB;

BEGIN {
    print "main::BEGIN\n";
}

##</code><code>##

$ perl main.pl 
ModuleB::BEGIN
ModuleA::BEGIN
main::BEGIN