package A;

use lib '.';
use B;

sub new {
    return bless {}, shift;
}
sub call_b {
    my ($self) = @_;
    my $b = B->new;
    return $b->call_rest_client;
}
1;

##</code><code>##

package B;

use REST::Client;

sub new {
    return bless {}, shift;
}
sub call_rest_client {
    my ($self) = @_;

    my $client = REST::Client->new;
    $client->GET('adfdsaf');
    return $client->responseContent;
}
1;

##</code><code>##

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

use lib '.';
use A;
use Mock::Sub;

my $obj = A->new;

say "Before mocking REST::Client->responseContent:\n";

say $obj->call_b;

# Mock the sub, and set a custom return value

my $response_client_sub = Mock::Sub->new->mock('REST::Client::responseContent');

$response_client_sub->return_value('{"a": 1, "b": 2}');

say "After mocking REST::Client->responseContent:\n";

say $obj->call_b;

##</code><code>##

Before mocking REST::Client->responseContent:

Can't connect to adfdsaf:80 (Name or service not known)

Name or service not known at /home/spek/perl5/perlbrew/perls/perl-5.26.1/lib/site_perl/5.26.1/LWP/Protocol/http.pm line 50.

After mocking REST::Client->responseContent:

{"a": 1, "b": 2}