Concering the correction - you could just append ->(). The null thing doesn't work if you get a key not contained in the hash, though. I'd do something like

my($favorite_color) = ''; # get some kind of input here

my $action = {
      'red'    => sub { 'the color is red.'    },
      'yellow' => sub { 'the color is yellow.' },
      'blue'   => sub { 'the color is blue.'   },
}->{$favourite_color} || sub { 'I do not know that color' };

print $action->();
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You can arrange the same semantics in different ways, of course. I chose it like this since I was aiming to keep the "default case" together with the rest.

Makeshifts last the longest.