JavaFan,
I am not sure this is a good way, but it is "a way".

use constant TEST_MODE => 1;
sub foo ($) {
    my $arg = shift;
    state %cache;
    return $cache{$arg} if exists $cache{$arg};
    if (TEST_MODE) {
        # ... something that would only run if cache failed
    }
    # ...
}
[download]

Of course TEST_MODE needn't be a constant - it could be anything that detected this was running under a test harness. There was a node a while back about marking code as deprecated that would issue a warning during testing but would function normally in production. See RFC: Devel::Deprecate for possible ideas.