Why mock doesn't work in this case?

[PerlOnTheWay]

try.t:

#! /usr/bin/perl
use strict;
use warnings;
use Test::MockModule;

my $father = Test::MockModule->new('Father');
$father->mock(alert => sub {
        print 'replaced by try' . $/;
});
do 'Son.pm';

Father.pm

package Father;

sub alert {
    print 'father' . $/;
}

1

Son.pm:

package Son;

use base Father;

sub new {
    bless {}, Son;
   *{'Father::alert'} = sub {
       print 'son' . $/;
   };
}
Son->new->alert();

When I run try.t,nothing is outputed

But if I delete the typeglob operation in Son.pm, I can get something outputed

why?