sub greeting {
    my ($verb, $object) = @_;
    my $sub = defined $object
        ? sub { print "$verb, $object\n" }
        : sub { print "$verb, ", shift(), "\n" };
    return $sub
}
my $hello = greeting('Happy Birthday', 'L~R');
$hello->();
my $bye = greeting('goodbye');
$bye->('Cruel World');
__END__
Happy Birthday, L~R
goodbye, Cruel World

##</code><code>##

greeting('Hello', 'Good Looking'); # prints immediately
greeting('Woah'); # Waits
...
greeting('Nelly'); # prints now that it has both arguments