Shorten code: merge arrays from hashes - perl

Question

I have a list of hashes and some of the hashes contain a key which provides an array itself.
my #cars = (
{ # empty car
name => "BMW",
},
{ # car with passengers
name => "Mercedes",
passengers => [qw(Paul Willy)],
},
...
)
It's pretty much like above but of course not with the stupid cars example:-)
Now I need to get a list of all "passengers" from all hashes including ones that do not even offer a passengers array.
In a 2nd step I need to retrieve unique entries from the list (actually the passengers are Perl object refs and I need every object once in the list)
At the moment I do:
my (#all, #passengers, %seen);
for(#cars) {
push #all, #{$_->{passengers}} if $_->{passengers};
}
#passengers = grep { ! $seen{$_} ++ } #all;
I would like to get rid of #all and throw the list of all passengers directly into grep.
Any suggestions?

Answer 1

my %seen;
my #passengers = grep { ! $seen{$_} ++ }
map { #{$_->{passengers} || []} } #cars;

Answer 2

It bugs me to create an array and a reference only to immediately get rid of it both (like cjm did), so I'd use
my %seen;
my #passengers =
grep !$seen{$_}++,
map $_ ? #$_ : (),
map $_->{passengers},
#cars;
or
my %seen;
my #passengers =
grep !$seen{$_}++,
map #{ $_->{passengers} },
grep $_->{passengers},
#cars;

Answer 3

Here's another variation. It uses List::MoreUtils::uniq. The %seen stuff is good to know, but unnecessary these days.
use List::MoreUtils qw<uniq>;
my #passengers
= sort uniq map { #$_ } grep { defined } map { $_->{passengers} } #cars
;
Of course, using my idiom of list_if, I would just do this:
my #passengers = sort uniq map { list_if( $_->{passengers} ) } #cars;
Where list_if is defined as:
sub list_if {
use Params::Util qw<_ARRAY _HASH>;
return unless my $cond = shift;
return unless my $ref
= #_ == 0 ? $cond
: #_ == 1 ? $_[0]
: \#_
;
return !ref( $ref ) ? $ref
: _ARRAY( $ref ) ? #$ref
: _HASH( $ref ) ? %$ref
: ()
;
}
It's been a useful idiom for cutting down on long-hand approaches to deciding whether to "stream" array and hash refs or not.