$ cat time.pl

use strict;
use warnings;

BEGIN {
  no warnings qw(redefine);
  *CORE::GLOBAL::time = sub { 4 };
}

my $time = time();

1;

$ perl -d time.pl

Loading DB routines from perl5db.pl version 1.28
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(time.pl:10):     my $time = time();

DB<1> n
main::(time.pl:12):     1;

DB<1> x $time
0  4

DB<2> x time()
0  4

DB<3> q