2014-03-20 15:53:01 dpchrist@p43200 ~/sandbox/perl
$ cat mock-system.pl 
#! /usr/bin/perl
use strict;
use warnings;
$| = 1;

package Foo;

sub foo { system @_ }

package main;

print Foo::foo("echo", "-n", __LINE__ . "\n"), "\n";

{
    package Foo;
    use subs "system";
    package main;
    *Foo::system = sub { print "mock system() called: @_"; 1 };
    print Foo::foo("echo", "-n", __LINE__ . "\n"), "\n";
}

{
    package Foo;
    use subs "foo";
    package main;
    *Foo::foo = sub { print "mock foo() called: @_"; 2 };
    print Foo::foo("echo", "-n", __LINE__ . "\n"), "\n";
}

2014-03-20 15:53:06 dpchrist@p43200 ~/sandbox/perl
$ perl mock-system.pl 
12
0
19
0
Subroutine Foo::foo redefined at mock-system.pl line 26.
mock foo() called: echo -n 27
2