gistfile1.pl

# Foo.pm
package Foo;
sub say { print "Foo\n" }
1;

# Foo/Bar.pm
package Foo::Bar;
sub say { print "Foo::Bar\n" }
1;

# ok.pl
use strict;
use warnings;
use Class::MOP;

my $class;

$class = 'Foo::Bar';
Class::MOP::load_class($class);
$class->say; # print 'Foo';                                                                                                                                                   

$class = 'Foo';
Class::MOP::load_class($class);
$class->say; # print 'Foo::Bar';                                                                                                                                              

__END__

results:
Foo::Bar
Foo

# ng.pl
use strict;
use warnings;
use Class::MOP;

my $class;

$class = 'Foo::Bar';
Class::MOP::load_class($class);
$class->say; # print 'Foo';                                                                                                                                                   

$class = 'Foo';
$class->can('say'); # add 'can' entry in GV stash? # MOP.xs Line 393                                                                                                          
Class::MOP::load_class($class);
$class->say; # NG                                                                                                                                                             

__END__

results:
Foo::Bar
Can't locate object method "say" via package "Foo" at ./ng.pl line 14.