package Foo::Bar;

my $privateFunc1 = sub {
  ...
}

my $privateFunc2 = sub {
  ...
}

my $privateFunc3 = sub {
  ...
}

if (our $TESTING) {
   # Expose private functions for testing.
   *_privateFunc1 = $privateFunc1;
   *_privateFunc2 = $privateFunc2;
   *_privateFunc3 = $privateFunc3;
}