Class::Mock.pm

use strict;
use warnings;

package Class::Mock;

use Sub::Install;

sub import {
    my ( $self, $target, $sub_masks ) = @_;

    my $mock_class = caller();

    ( my $name = "$target.pm" ) =~ s/::/\//g;
    $INC{$name} = 1;

    {
        no strict 'refs';
        push @{"$target\::ISA"}, $mock_class;
    }

    Sub::Install::install_sub({ code => $sub_masks->{$_}, into => $target, as => "$_" }) for keys %{$sub_masks};

    return;
}

1;

something::mock.pm

use strict;
use warnings;

package something::mock;

use SUPER;

use lib '../../../lib';

use Class::Mock something => {
    new => sub {
        my ( $class ) = @_;

        my $obj = $class->super('new')->( @_ );
        return $obj if $obj->message;
        return;
    }
};

sub new { return bless {}, $_[0]; }

sub message { 1 }

1;