This is what the side_effect() functionality does... allows you to do stuff (eg: modify an object) when there's no need for a return. Here's an example:

use warnings;
use strict;

package Thing; {
    sub new {
        return bless {}, shift;
    }
    sub modify {
        my ($self) = @_;
        $self->{modified} = 'modified by original sub';
    }
}

package main;

use Data::Dumper;
use Mock::Sub;
use Test::More;

my $m = Mock::Sub->new;
my $thing = Thing->new;

my $modify_sub = $m->mock('Thing::modify');

$modify_sub->side_effect(
    sub {
        my $obj = shift;
        $obj->{modified} = 'modified by mocked sub';
    }
);

print "before mocked sub called...\n\n";

print Dumper $thing;

$thing->modify;

print "\n\nafter  mocked sub called...\n\n";
print Dumper $thing;

print "\n\n";

is defined $thing->{modified}, 1, "obj was modified ok";
like $thing->{modified}, qr/mocked sub/, "obj was changed by mock";
is $modify_sub->called, 1, "mocked sub called ok";

done_testing();
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Output:

before mocked sub called...

$VAR1 = bless( {}, 'Thing' );


after  mocked sub called...

$VAR1 = bless( {
                 'modified' => 'modified by mocked sub'
               }, 'Thing' );


ok 1 - obj was modified ok
ok 2 - obj was changed by mock
ok 3 - mocked sub called ok
1..3
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So, there's no return anymore. The side_effect() code reference will get all parameters passed in as they were sent in to the real sub call (in this case, $self, as it's the only param on the method call. We then have the side effect add a new hash key to itself and assign it a value. After side effect is complete, the main object is updated just like the original function would have done, without having to call the real function.