I recently needed such an abstraction as well. I had a hierarchy of classes with lots of class methods, which I'm in the process of refactoring into another shape. Since I didn't know exactly how yet, I started with the simplest thing that could possibly work:

package Model::Classloader;

sub new { my $pkg = shift; return bless {@_}, $pkg }

sub class {
    my $self = shift;
    my $class = shift;
    my $fullname = $self->{namespace} . "::$class";
    eval "require $fullname;";
    return $fullname;
}
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The usage is pretty simple: you instantiate a Model::Classloader object, giving it a namespace attribute, and then you can repeatedly call class() on it to get the appropriate package name.

Here's an example for your code:

package MyApp::Frontend;

use Module::Classloader;
our $C = Module::Classloader->new(namespace => 'MyApp::Adaptor');

sub get_report {
    $C->class('Report')->get_report($report_id);
}

package MyApp::Adaptor::Report;

use Module::Classloader;
our $C = Module::Classloader->new(namespace => 'MyApp::Backend');

sub get_report {
    # some business logic such as check if report exist, etc. 
    $C->class('Report')->get_report(...);
}
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This does pretty much exactly the same as your original code, but now the package names are abstracted away. When you decide you need more functionality, you can get it by simply extending the class loader.