sub print_me {
  my $arg = shift || "Hello World!\n";
  print $arg;
}

%hash = (
  no_arg   => \&print_me,
  with_arg => sub { print_me("Hello World, again!\n") },
);

$hash{no_arg}->();
$hash{with_arg}->();

__output__

Hello World!
Hello World, again!