You don't need to overload an inbuilt function IF you define one of the same name and call it with the ampersand prepended, i.e. &time will execute a user-defined subroutine named time if it exists. Otherwise, you can check out dchetlin's tutorial Overloading '=' for some info on how to overload inbuilt functions.

As to the first problem: you can wrap require Time::Hires in an eval inside a BEGIN block, and if that eval fails, print a warning (if appropriate) and set a flag that tells your program to use the internally defined routine. If the eval succeeds, call import Time::Hires and you'll have the symbols imported.

#!/usr/bin/perl -w

use strict;

use vars '$internal';

BEGIN: {
     eval "require Time::Hires";
     if ($@) { # if it failed
          print "Can't load Time::Hires, falling back to own routines\
+n";
          $internal =1;
     } else {
          import Time::Hires;
          $internal = 0;
     }
}
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