You can change a method at runtime with a scoped application of { local *Apache::FakeRequest::lookup_uri = sub { '127.0.0.1' } ...}

Here's an example using CGI.pm:

use CGI;
my $q = CGI->new({foo=>'bar'});
print $q->param('foo'),$/;
{
    local *CGI::param = sub {};
    print "Empty", $/ unless defined $q->param('foo');
}
print $q->param('foo'),$/;
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That does not change the behavior for just a single object, but for all objects of the class within the local dynamic scope. As othermonks said, subclass for specializing a single instance.

After Compline,
Zaxo