I wrote Mock::Sub several years ago, apparently waiting for this exact problem!

use warnings;
use strict;
use feature 'say';

use Mock::Sub;
use Tesla::Vehicle;

my $mock = Mock::Sub->new;
my $car = Tesla::Vehicle->new;

my $speed_sub = $mock->mock('Tesla::Vehicle::speed');
$speed_sub->return_value(60);

# No matter how fast I'm going, we'll always return 60!

say $car->speed;
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