sub say_hello
{
  my $opts = shift;
  my $hello_sub = $opts->{hello_sub} // sub { my $o = shift; say "hello, ", $o->{who} // 'world' };

  $hello_sub->($opts);
}

say_hello(); # says "hello, world"
say_hello({ who => 'temporal'}); # says "hello, temporal"
say_hello({ hello_sub => sub { my $o = shift; say "wazzup, ", $o->{zup} // 'homie' }, zup => 'bro' }); # says "wazzup, bro"