#!/usr/bin/env perl
use strict;
use warnings;
my $total_sleep_time = 0;
BEGIN {
*actual_sleep = *CORE::GLOBAL::sleep; # ?????
*CORE::GLOBAL::sleep = sub { $total_sleep_time+=$_[0]; actual_sleep($_[0]) } # << line 7
}

# use this module and the other
# ...

sleep(2);
print "total sleep is $total_sleep_time\n";