#!/usr/bin/perl

use strict;
use warnings;

my $total_sleep_time = 0;

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

print  "\$]=$]       \$^V=$^V\n";
sleep(2);
sleep(1);
sleep(1.3);
print "total sleep is $total_sleep_time\n";

$ time -p ./sleep.pl 
$]=5.024001       $^V=v5.24.1
total sleep is 4
real 4.00
user 0.00
sys 0.00