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;