C:\Users\Bill\forums\monks>type Existing_module.pm
use strict;
use warnings;
package Existing_Module;

sub Do_Something {
    print "Doing something.\n";
    sleep(3);
}
1

C:\Users\Bill\forums\monks>type bliako.t
use strict;
use warnings;
use lib '.';
use Test::More tests => 2;
use Existing_Module;


our $total_sleep_time;
BEGIN {
    *actual_sleep = *CORE::sleep;
    *CORE::GLOBAL::sleep = sub (;$) {
        $total_sleep_time+=$_[0];
        actual_sleep($_[0])
    }
}
sleep(2);

is($total_sleep_time, 2, 'Local');

Existing_Module::Do_Something();    # Sleeps for 3 sec.
is($total_sleep_time, 2+3, 'Module');



C:\Users\Bill\forums\monks>perl bliako.t
1..2
ok 1 - Local
Doing something.
not ok 2 - Module
#   Failed test 'Module'
#   at bliako.t line 21.
#          got: '2'
#     expected: '5'
# Looks like you failed 1 test of 2.