Overriding a module that is used by a program I'm testing

I am revising a Perl program and I wanted a test harness that could run the original version of the program (call it launch_rockets.pl) and collect the standard output, but somehow skip the system calls that occur inside launch_rockets.pl. The following code successfully overrides system inside launch_rockets.pl:

use subs qw(system);
my $SYSTEM_SUCCESS = 0;
sub system {
    print "***\n";
    print "system @_\n";
    print "***\n\n";
    return $SYSTEM_SUC开发者_如何转开发CESS;
}
local @ARGV = @test_args;
do 'launch_rockets.pl';

So far so good. But launch_rockets.pl also contains

use Proc::Background;

and later

Proc::Background->new('perl', 'launch_missiles.pl');

I could copy launch_rockets.pl into a sandbox where Proc::Background is replaced by a stub, but I was wondering if there was any override strategy that would be effective inside a do FILE call in the file's original environment.

use lib '/my/test/library/path';

lib prepends the directory to @INC, so /my/test/library/path/Proc/Background.pm will be the file that gets loaded. Put whatever code you want in there.

Another alternative would be:

{
  package Proc::Background;
  ... # Put stub code here
} # end of package Proc::Background
$INC{'Proc/Background.pm'} = 1; # Make Perl think Proc::Background is loaded