my $logger;
use constant DEBUGGING => !!$ENV{DEBUG};
sub log_this (&) {
   DEBUGGING or return;
   $logger ||= Logger->new;
   $logger->log($_) for shift->();
}

log_this { "Hello world" };
log_this { "Hello", "world" };
log_this {
   my $data = Expensive->method_call();
   "DATA: $data";
};